When you are on defense get close to the person with the ball and press the Y button.It should knock the ball out of their hands and if it does retrieve it. Sometimes you might foul them our miss them.
If you dont know how to slam this hint is for you. Slaming is easy. First you use your Turbo and then when you are close up to the basket press B. (Sometimes it will do a lay-up, depending how tall your player is).