This has to be one of the simplest yet most effective S Mount Arm Bar defenses in BJJ.
Here are 2 great options to deal with the top Half Guard Switch Base (when your opponent is facing your legs).
The Side Smash Pass is one of the most demoralizing guard passes in BJJ. It not only puts your opponent in a horrible and uncomfortable position
Escaping the Back is one thing, defending and preventing the hooks from being inserted in the first place is another.
Here's an awesome Open Guard Drill to improve your transitions and control. The Open Guard is one of the most difficult guards to master in BJJ