Muzzy,
Along with smacking your arm, you'll notice that by shooting with a locked arm, which creates a TENSE arm, you will shake much more. Try clenching your fist, if you clense hard enough, kind of like when shooting a bow, you fist/arm will shake. It's the same whether it's you arm, shoulder rolled up to compensate for a long draw, hand clenched around the grip, face stiff, or whatever. When you clench, you shake.

Good point!

Jeff...U.P. of Michigan