Post defense may look different depending on the specific players and post areas in question, but there are some basic principles that are commonly-understood to basketball coaches, analysts, and players everywhere.
The first step of playing post defense is to deny the ball to the post. This means getting in front of the post player to block a possible entry pass from a teammate. If a pass is made that the defender cannot get to, they must adjust their position while the ball is in the air.
In the low post, the post player will usually try to back down the defender before spinning to get an open shot at the basket. Once an entry pass is received, giving some room to the post player will not allow them to push off for a quick fadeaway or feel the defender's body lean a certain direction. Post defenders in this situation should use a strong forearm or hand on the back of the offensive player, as well as bending knees and having active hands. The armbar prevents the post player from backing the defender down too easily, but also conceals the defender's body position.
If the post player faces the basket, the defender must get as close as possible, forcing the offensive player to dribble a certain direction, take a difficult contested shot, or pass the ball out of the post.
Defending the High Post
While defending the high post is very similar to defending the low post, there are a few differences. An offensive player in the high post is unlikely to try and back down a defender from such a far distance, but there is a bigger emphasis on what goes on around the post player. Because the high post is in the middle of the frontcourt, there are more options for the post player in this position. The post defender must be aware of the cuts, screens, and other passing options presented by the offense from this area.