And you are not to stab with thumb on guard, or you will break your thumb. The thumb is on the side of the knife to allow both slashes and more effective back cuts, with slight adjustment to hammer grip for a thrust, followed by a good crank to induce shock. Even when held vertically, using the thumb during a stab is a good way to end up with only one useful hand. Forward curved guards should be for keeping hand off blade and catching blades, not for thumb rests during a stab.

(This where Sgt Lofty orders a stubborn PFC to stab a tree 50 times using thumb. hahaha) Actually, there are as many ways of knife fighting as there are knife fighters. And old farts stick with what they were taught, and what they taught.