When Cribby does the rig flip and the old back hand becomes the new front hand, as he reaches under he grabs the other side of the boom with an underhand grip instead of an overhand grip, wondering how many people do that.

When he does a boom shaka, sailing out clew first, he extends the front arm throwing the rig forward while letting go of the back hand which enables it to slide towards the mast clamp, so he's not really sliding his hand back in a boom shaka he's pushing the rig forward and letting the boom slide thru this back hand.
I have one of those T boom bras with a small mast pad connected to the boom bra, this keeps me from grabbing the mast.