Press triangle during a game brings up the Movement menu. If your team has the ball, you will have options: Auto, Manual A, and Manual B.
The computer controls player movement in Auto, the default setting. Manual A and Manual B allow you to control the player with the ball. In Manual A mode, direction of movement corresponds to the mini-map; swimming left always directs the player toward the enemy goal, while swimming right directs him toward his own goal. In Manual B mode, directional pad directs the ball carrier toward it.
When you choose a Manual option, the Formation menu also appears. The default number of formations your team starts with is four, and you can earn more formations as your team level increases. The four standard formations are Normal, Mark Mode, Right Side, and Left Side.
Normal is basically a zone defense, where your players go after the ball carrier once he gets close, and pursue him for as long as they can keep up with him.
In Mark Mode your players cover only the opponent assigned them in the pre-match setup screen, regardless of whether that opponent has the ball or not.
With Ride or Left Side formations, your players play a zone defense on that side of the playing area. They leave the other side relatively undefended, but no one can get through the side they are on.