Being able to score in the post is all about being able to make a move really well, and then also having a counter to that move as well. If you are trying to go to the middle and the defender cuts you off and takes that away from you, you need to be able to counter and finish. This is where the drop step move comes into play.
You are going to use the defenders aggressiveness to take away the middle against themselves. As soon as the defender rotates to the middle to cut you off, you are going to drop step to the baseline and finish instead. To make this move successful you need to be able to sell that you are going to the middle and get the defender to cut you off. Also, you need to make sure that you have the proper footwork and a good angle to make the move. This is one of the basketball fundamentals for post players, and this training video will teach you how to do the move.