Yeah, it's just setting the bullets so they are fitted back properly to the rear of the magazine so they feed right.
You can either smack it on your hand or on your helmet. Don't tap it on your head without a helmet though, it's a dumb and painful thing to do.
I wonder is the sadistic bastard who invented the AK magazine put that sharp crease on the back just to make sure people cut their hands setting the mags.
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