The trigger finger hands, they are the only hands without wrist movement for added strength.
The thumb in particular is fairly flexible and can be moved a little. The first time I managed to get it on the thumb was on the wrong side. The best way I found was to try from the side of the handle/gun rather than from the back, and have the finger first. Tried doing pics, but camera ran outta juice for that one... Not like the pic above, but if we use the gun as a point of reference, so the wrist is pointing away from the camera and the index finger is pointing at you, but the gun is in the same spot.