Here’s what I learned from some pretty good teachers:
A controlled pair is two shots in rapid succession. The entire sequence – sight picture, alignment, controlled surprise break, occur for each shot. Skill level and the size and distance of the target determine the speed.
The Hammer is an exercise in timing. When a handgun is fired the muzzle rises then comes down BELOW the line of sight, then back up (if an isometric tension grip and stance are used such as a Weaver, Chapman, or some variations of an Isosceles). For the Hammer you find the timing to let your second shot off as the alignment is crossed on the way down. It doesn’t really work if the grip and stance aren’t consistent. The more consistent you are and the better your timing, the greater the range you can place your shots together. Even relative novices can place their shots within four or five inches at three yards if they observe the basics. The magnificently skilled seem to be radar guided. I saw a demonstration by a master in the early eighties. Twelve yards, two and one half to four inches apart, three times, WITH HIS EYES CLOSED. He did it as a demonstration of muscle memory, not to show off.
At the highest skill levels, the two techniques seem to merge. These shooters always seem to control every shot. I believe that the rise of the compensated pistol in competition has led to taking the hammer out of the repertoire for most competition shooters.
The Hammer might not have much use anymore, but it was a great stunt and loads of fun to learn. I burned a lot of powder, made a lot of dirty brass and had a ball learning.
"Smooth is five sixths of speed" - Ray Chapman, teacher and first practical pistol world champion.