I have Mantis experience, Believe it or not, and it is in 7* mantis, believe it or not.
That Pincher claw or Hook helps train the forearm for blocking and also keeping a close guard for yourself when being attacked(ie pulling that elbow in). It also helps to train that forearm for adhereing to a punch when it is thrown. You use that claw/hook to block and draw in the punch. Its like you are pulling downward and in once you block the punch. From there you are either going to punch to their face or you are in close to subdue the opponent(ie: head lock or choke). The two fingers that are being braced by the thumb just aid in that grasp and also can brace the fingers for a poke/Strike to the eyes.
Peace,TWS
It makes me mad when people say I turned and ran like a scared rabbit. Maybe it was like an angry rabbit, who was going to fight in another fight, away from the first fight.