It makes your hand tremble like you've got the DT's. By gripping the controller the right way, the vibrations click the button at higher frequency than you can click it the normal way.
Not to mention the bicep muscles are going to be stronger and easier to do the vibration with than ANY muscle in your hands.
At least, that's the way I figured it out-- by locking my hand and lower arm muscles tight, then using the strongest muscle in my arms (the biceps) to do the actual work.
