1. The forward arm eliminates "dipping" or dropping the hands and trailing shoulder to lift the ball with a "looping" type swing. If you "dip" with the AST, you hit the back of the arm. It forces you to take the bat straight down to the ball, leveling the swing at the point of contact.
2. The forward arm also pivots and rotates to place the ball on the inside or outside of the strike zone. Then, the arm points in the direction to drive the ball based on pitch location (i.e. pull the inside pitch, go with the outside pitch to the opposite field . . . "Hit the ball where it's pitched").
What is the Purpose of the Outside Barrier?
The outside barrier eliminates "casting". It keeps you form swinging "long" and helps you "keep the hands inside the ball". If the bat or arms are extended prematurely the bat head will slap the flexible upright barrier post. For years coaches have set a tee adjacent to a fence or screen to force hitters to compact their swing.