There are a lot of different ways that the defense can guard a ball screen, and it is important that the ball handler is able to quickly recognize what their defender is doing, and then do the right move to make them pay. For example, if the defender guarding the ball handler goes under the ball screen, then that is the perfect time to pull up behind the screen and shoot the uncontested shot.
This is exactly what the read and react guard defense ball screen series drill is designed to do. There are going to be 3 different ways that the defender can guard the ball screen, and according to how the player is guarded, the player must read it and then react accordingly to it to make the right play. Basketball drills like this on are great for practicing live game situations, and learning to make defensive reads.