cowboy decock

(verb) the act of decocking a handgun with an exposed hammer by pulling the trigger while the thumb of one's firing hand rests on the hammer, stopping the hammer's motion, then slowly allowing it to move until it rests at the base of its travel without striking the firing pin with sufficient force to ignite the primer of a chambered round.

I was afraid Jeb was going to put a hole through the floor of my car when he cowboy decocked his revolver.