Do not use extra force to make a more powerful shot. Always use the same force as you normally play - This is for a soft delicate touch or a high powerful shot. If you want a powerful shot, just pull some extra length on the final backswing (cue tip almost on top of your bridge). If you want a delicate touch, just pull maybe just couple of inches on the final backswing. Always remember to check the cue for alignment with the 'line of aim' and delivery it smoothly. No extra muscle necessary.