2 Part Technique - Never allow your opponent to gain an underhook on you while playing from Half Guard, and certainly don't let him clasp his hands together around your head and arm. With that said, if you are trapped under this dominating control, utilize a half-bridge to open up enough space to form a frame under his neck. Aftering pumping yourself up enough to nudge open room for your own underhook, turn onto your side and scoop up his leg with a Butterfly hook for the Hook Sweep or Guard Recovery. If his weight is too far away to undo his underhook, bridge into the other direction and undo his crossface first, and then set your Butterfly hook with an underhook. If he places you in Quarter Guard, with his knee outside of your guard, bump and frame again for the underhook and then push his knee back into your Half. Oh, and eat your Fruits & Vegetables.