In his drills, you're left side is looking for its center of balance. You are working on proprioception and knowing where your body is in an unstable environment.
Foley is also big on mental balance as well. In his interview describing his coaching philosophy for Tiger Woods:
It's a minimalist approach. I remember reading in Mike Hebron's book, "The Art and Zen of Learning Golf," it says there are things that you cause to happen and things that you allow to happen. When you start trying to cause what's already allowed to happen, you're going to run into problems. There's a catalyst to certain things that you build in the backswing, and in the downswing there are things that are a catalyst to what happens in the through swing.
“You don’t really show Stephen Ames or Tiger Woods what to do,” says Foley, who made no secret of wanting to coach Woods and got the chance this year. “You paint a picture for them in their mind and they practise it until they feel it in their bodies.”
So, we are looking at a mental balance as well. A kind of open focus, if you will, where you are giving up control and giving up forcing your body to do something in favor of an awareness.
Sean Foley Golf Channel Tip on Balance | Golf Progress
PGA Tour Interview With Tiger Woods