Watch Hickok45 (a large man with decades of experience shooting Glocks in all calibers) starting at the 2:40 mark. Every time you see his support hand re-adjust, what you're witnessing is the fact that, under recoil, during strings of fire, his support hand has slipped off of its ideal position, and the support grip must be adjusted back to where it's supposed to be. It's subtle, but watch closely and you will see it happen several times during strings of fire.
I guess he needs more protein in his diet, right? And I'm willing to bet he didn't even know he was doing it, because it had become second nature to make those readjustments whenever his support hand slipped off its ideal lock-up position during strings of fire. This is more likely to occur with a Glock 22 than with a Glock 17.
