Games : How can I escape being pinned by monsters?

I've been pinned down multiple times, I've also escaped in some cases, sometimes not. Since I have no clue how to escape I just buttonmash every button I can reach without moving my hands.

Since I got a quest today to beat up some huge crab who kept grabbing me, I need a solution to consistently get away if I want to beat that beast again. How do I evade being chewed on?