FMJ rounds can sometimes fail to cause sufficient damage (at least when compared to expanding bullets), requiring more hits and better shot placement.
The term double tap is now used to describe the more general technique of firing two rounds quickly and accurately to disable an opponent.
Double taps are an integral part of the El Presidente combat pistol shooting drill developed by Jeff Cooper during the 1970s and published in the January/February 1979 issue of American Handgunner.
According to a U.S. Army training manual, "There is a natural arc of the front sight post after the round is fired and the recoil kicks in.
The soldier lets the barrel go with this arc and immediately brings the front sight post back on target and takes a second shot.