Because they are already tired from the running test before that .. They have about 30 minutes rest at most before that game.
The one touch ban means that the opposite side have to pressurized to win the ball.. Its a very tiring process.. pressuring, back to formation, pressuring .. Tatsumi also bet on the fact that the sub team have their pride and will try to win the ball at all cost. Also if he allow the one touch game from the very beginning, his team will get scared and will lose the ball more easily, but by telling the boys to keep the ball first, he ensure that they are more settled while the other team gets more irritate and tired
There's a lot of psychological play involved in his strategy for that single training match.. Something that only possible after he spend his whole night watching the team on the CD.