Why would you want to move your thumb to the under side of the safety once you have acquired your sight picture? To do so will disturb your sighting, interrupt your concentration, and slow your shot. Moving the 1911's safety to the fire position requires no change in grip, and no disturbance of the sights. Once you have made your shot(s) there is plenty of time to move your thumb to the underside of the safety to lift it into the safe position. An additional advantage of keeping the thumb above the safety is that if your gun is equipped with an ambidextrous safety, the fleshy part of your hand can push the safety upwards into the safe position when you do no want it to. Downward pressure on the safety by the thumb of the strong hand prevents this from happening.