Unless the player you intend to post-up with has a 'post control' rating of 80 or above, attempting to back down in the paint is likely to result in turnovers and missed shots. However, in the right hands, it can be a devastating offensive tactic, one resulting in many easy baskets. Press L2 in the paint to turn your back on an opponent and begin the post-up animation. Take time to learn a few basic moves. The most useful is probably the 'post spin' (a full rotation of the right analogue stick in either direction), although it's best attempted after calling for an isolation play to clear space under the basket. It's best not to attempt too many post moves in-game before mastering them in practice mode, as inexperienced play down low can result in a bucketload of turnovers.