Back cut aka "Bowie's deadly back cut". It can come from anywhere and go anywhere, independent of initial blade move, and too fast to counter. As fast as you can go from thumb up to thumb down, left or right, up or down. Three can be done in one thrust or parry. All the force of the blow is concentrated on very tip (razor sharp tip, or should be) which acts as a raking talon. At least, I am guessing that is what Bill was showing. And could be wrong as wrong can be.

PS- Was laughing after reading what I wrote. Was thinking of the Crocodile Dundee "now...THAT'S a knife" scene, and how unintentionally accurate the screen gag writers were in basic knife as well as the cuts to the mugger's jacket.