What is tempo?
The non-musical definition of tempo is something like: “A characteristic rate or rhythm of activity; a pace”. In Magic it's basically a term that refers to your ability to utilise various aspects of the game, such as mana and life totals, to your advantage. Imagine a theoretical game of Magic where each player had a deck full of lands and 2/2 creatures that all cost 2 mana, but that can't block. If you go first, (assuming you draw a reasonable number of lands and creatures) you win. You attack for 2 on turn three with your first guy. Your opponent attacks you back. You attack for four on turn four with two guys. Your opponent attacks you back. You attack again on turn five, and you kill your opponent on turn six, the turn before you die. Why did you win that game? Obviously, it's because you went first. But in terms of tempo what did that gain you? Quite simply, you had access to two mana the turn before your opponent did. That enabled you to establish tempo over your opponent, and with those theoretical decks there was nothing they could do to get it back.
I'm sure many (if not all) of you have lost a game of Magic the turn before you would've killed your opponent simply because you lost the coin-flip and were forced to go second. That was a loss to tempo. The above example illustrates perfectly why it is almost always correct to go first in a game of Magic, and certainly in the Kamigawa limited block. There are some limited formats which are slow enough that card advantage is more important and in those formats you might choose to draw first, but those formats are few and far between, and the Kamigawa limited block certainly isn't one of them.
It all comes down to mana
As I just mentioned, tempo can apply to different areas of Magic. At its most basic level it simply applies to your mana. If you have a Grizzly Bears and your opponent has a Gray Ogre then you both achieve the same thing when casting them, but you gain a mana. Sometimes this will mean you get an advantage because your creature comes into play earlier and is able to attack first, sometimes it means you gain an advantage because you're able to use that extra mana to do something else. You've gained tempo. Sometimes of course it won't mean anything if you're unable to use that extra mana, but the potential is there.
Similarly, sometimes you can use a cheap spell to nullify a more expensive spell of an opponent. Your opponent spends their sixth turn casting Keiga, the Tide Star; you spend your sixth turn casting Cage of Hands on Keiga. In that situation you've gained three mana over your opponent and hopefully you'll be able to use that mana to perhaps cast an extra creature or spell. Once again, that is tempo you have gained.
This is the whole reason why people talk so extensively about mana curve. If your deck has a good mana curve with many things to do for small mana costs then typically you will have a greater chance of being able to utilise your mana in the early turns of the game, and also have a greater chance of using any spare mana you might have in later turns. Decks with low mana curves have them because they want to establish tempo in the early part of the game and use it to help them win.
The other thing that impacts tempo in a game of Magic is the life totals of the players. Typically whichever deck is the more controlling deck in a particular match-up has to sacrifice its life total early on before it can hopefully try and restore the tempo balance with it's more powerful spells later. The aggressive deck would start out gaining tempo with such cards as Goblin Cohorts and Villainous Ogres and so forth, but once the controlling deck gets it's more powerful spells online it would hope to be able to stabilise the board and restore the tempo balance. The aggressive deck simply wants to reduce its opponent's life total to zero before the controlling deck is able to re-establish that balance. If it fails, then it has to fall back on cards which are capable of dealing those last few points of damage through evasion such as Nezumi Cutthroat or other methods like Frostwielder and Devouring Greed. If the controlling deck has had to sacrifice too much tempo early on then its life total will be low enough that the aggressive deck's finisher wins it for them.
Now historically the colours of black and red have been the colours of aggression and blue and white are the more controlling ones, but that doesn't have to be the case. If the blue-white deck starts off with Isamaru, Hound of Konda, Kami of Ancient Law and Soratami Rainshaper, then it would be the black-red deck that was probably behind on tempo and that would have to try to re-establish it. No matter what colours you draft and play, it's always important to determine whether you intend to be the aggressor both during the draft and during a game.
There are often cards that effectively trade life for mana and occasionally vice versa. These can be very useful in establishing and maintaining tempo. Spells like Vendetta and creatures like Takenuma Bleeder allow you to trade life points for a cheaper mana cost. As long as you can maintain the tempo they help establish, the cost of a few life points will be completely irrelevant. Similarly you can also have cards that work in the opposite way, in that their primary role is to trade mana for life. Cards like Kaijin of the Vanishing Touch illustrate this. This card is never going to help you win the game. It won't kill your opponent or run them out of cards. However what it does do is basically trade two mana for a number of life points. In most situations it will completely negate a two-power attacker and force the opponent to re-cast anything they do attack with. You'll probably get a net gain of at least two mana from this while preventing some damage at the same time. Of course you do also risk the chance of it being useless if your opponent kicks off with Nezumi Cutthroat and Soratami Rainshaper but usually cards like Kaijin of the Vanishing Touch go a long way in preventing the aggressive decks getting a big head start on you.
Tempo and tricks
If we give the green player the first turn play then you would play the Kami on turn two, and they would play their Kami of the Hunt on turn three. You can still cast the Houndmaster on turn three and attack with both of your guys but in that situation the green player can trade the two Kamis off without any fear of combat tricks. They might then be able to follow up with an Order of the Sacred Bell on their next turn that would start to swing the damage race into their favour.
Lets modify the situation a little and change the green cards. This time they are going first and they start with an Orochi Sustainer on their second turn. You follow with your Kami of Ancient Law. They then make the Order of the Sacred Bell on turn three. You can make your Houndmaster and attack with it, but then you take four damage when the Order attacks you back, and maybe your opponent follows up with a Feral Deceiver (with mana open to check the top card of their library) or a Frost Ogre. In this situation you've had exactly the same draw but your opponent has established a much greater tempo advantage than you. Perhaps your only fourth turn play is a Kami of Fire's Roar. In that situation you'll be in a lot of trouble when your opponent attacks with their guys on their fifth turn as you're now the one who is tapped out and vulnerable to combat tricks.
These examples aren't uncommon game situations and they're quite typical of the first few turns of Limited Magic in this block. Hopefully they do a good job of illustrating why tempo is important and why you should care about it when drafting, building your decks, and playing your games.
Gaining and losing tempo
Equipment is one subset of cards that can be big losers in tempo terms. No-Dachi is a perfectly good example of this. It costs two mana to cast, and three to equip. Lets say for example you cast it on your fifth turn and then equip it to your fourth turn creature and attack. Any removal spell from your opponent gains them a lot of tempo there as you basically lose the casting cost of the creature and the equip cost of the No-Dachi in return for them casting their spell. Typically you'll be losing three or four mana in that trade. If they have a bounce spell the situation is almost as bad as you'll have to recast your fourth turn creature and then wait another turn before you can equip No-Dachi again. That's a lot of time and mana spent getting back to the exact same situation you were in two turns previously. Now there are some equipment cards like Umezawa's Jitte that are powerful enough that you don't care about the loss of tempo and others that are cheap like Lightning Greaves or Ronin Warclub so that they don't impact your tempo in as big a way.
Card drawing effects or anything that doesn't actually affect the board would be a loser in tempo terms. Counsel of the Soratami isn't a good card to have to play on the third turn as you may find the loss of tempo overwhelms you before the extra card has time to impact the game. Consider a situation where you're going first with an opening hand of:
You lead with the Akuba and the Ronin and hit your opponent for two on your third turn. They untap. Of all their possible plays, what's the one that you most want to see? It's pretty much either Counsel of the Soratami or Kodama's Reach. Either of those plays lets you untap, attack for five and (assuming you've drawn a land in three turns) drain them for two with the Akuba and cast the Deceiver. Your opponent would be on eleven life, facing down seven power's worth of attackers and you would have a removal spell in hand. That's probably game over a lot of the time. And all because their third turn play didn't affect the board.
Expensive reactive cards are also a big loser in tempo terms most of the time. Mostly I'm thinking about cards like Second Thoughts and Chastise and anything that requires you to sit back with a lot of mana untapped and do nothing. If your opponent can correctly read your hand they can play around either of these by simply not attacking and instead continuing to develop their board position. They can wait for a point in time when they either have an answer to your spell or when it's simply not as relevant for them to lose an attacker to an expensive spell. Meanwhile you fall farther and farther behind as they keep casting more threats and you have to keep holding your mana open for Second Thoughts.
It's even worse if you go first. They play their Akuba on turn two, and you follow with Prison on turn three. Now they can't attack at all if they want to cast the Ronin and if you make a suitable blocker they'd again have to skip an attack step in order to deal with it via their Befoul. In this situation Ghostly Prison gains a huge amount of tempo for you.
This example also relates perfectly back to my point about being aware of whether or not your deck expects to be the aggressor or not. If you lead with Goblin Cohort and Kami of Ancient Law how useful is that Ghostly Prison on turn three then? Not very. Ghostly Prison is one of a few cards whose sole purpose is to create tempo. If you already have the tempo advantage anyway then it literally does nothing and should not be included in your deck.
It's important to recognise the distinction between card advantage and tempo advantage. The aforementioned Ghostly Prison is a perfect example. It doesn't do anything by itself; it doesn't kill a creature or attack your opponent. In effect, all Ghostly Prison does is allow you to basically trade the card for your opponent's mana, but then that is what tempo is all about. Quite often tempo doesn't care about card advantage. Bounce spells can and often should be used just to create a tempo advantage, even though it's a loss of card advantage.
The more powerful cards in the game are those that can both generate tempo and card advantage at the same time. From this block Hideous Laughter is a good example of this. If your opponent starts off with Frostling, Kami of Ancient Law and Kitsune Blademaster then not only does your Hideous Laughter gain you a three-for-one card advantage but it also steals a tremendous amount of tempo from your opponent too. You've wiped out their first three turns for just four mana. Once again though, if you expect your deck to be the aggressor and the one that establishes tempo early on, then Hideous Laughter loses a lot of its power.
I hope I've explained the concept of tempo clearly enough. This article is not intended to be a comprehensive study of the subject, merely a useful primer for those of you who feel you wanted a better understanding of the concept. Hopefully it's given you some insight into why tempo is important and the different ways you might be able to establish it or steal it back from an opponent.