The crab scratch was created by QBert in 1995. He saw DJ Excel do a twiddle scratch, which is a two-finger version of the crab scratch. QBert expanded on the idea and created a scratch that used all four fingers.
Crabs are much like a multi-click flare, but the thumb holds the fader closed like a spring while the other fingers click against it in sequence.
Wondering how this works? Check out the description.