It's tension created from the shoulder muscles, upper back, and pecs. Think about when you're doing a sit up: your hip muscles, lower back, and core help you sit up and slow going back down. It's these same muscles that keep you balanced when you're sitting and standing because managing balance where your center of mass acts upon gravity requires less work than at the extremities.

The same principle applies to a handstand, but it is instead your shoulders, pecs, and upper back. I was being a bit broad when I said shoulders, sorry.