Mr. Ridgewalker nailed it. The second click when a properly timed cylinder locks just before the revolver fires is a great way to simulate single action with a double action only revolver like my 642-1.
One of the Range Masters at the Pima Pistol Club first clued me in. To speed the process, he suggested using enough finger on the trigger that you can just touch the thumb on your shooting hand and know when the cylinder is about to lock. Follow with a gentle squeeze to fire.
He is a retired LEO and more of a Colt revolver fan, but he says that technique was worth many points during his qualification shooting. You know, back in the days when just about all peace officers carried revolvers.
It works for me! This target was shot one handed and is easy to duplicate.