#1 In this counter for a hand shake, students will commonly press against the tip of the thumb & then try to lift the thumb away, thus releasing the attacker’s grip. Unfortunately this approach doesn’t work.
#2 Instead, press the tip of attacker’s nail at a 45 degree angle, pushing his thumb down towards the base of his thumb. Do not try to lift the thumb. [Notice the positions of my second & third fingers a the base of his hand. They’re there for support.] The pain will force attacker to release his grip & you can slide your hand out. If you maintain pressure on the thumb tip you can either execute a simple takedown, go into a variation of hand throw, or a variety of other techniques.