[Tijeras_Slim]

He pulled the trigger before he cocked the gun. When he cocked the hammer he thought it would be held by the sear. Let go of the hammer and the gun fires.

Back in the day, pistoleros would modify Colt SAA’s by tying back the trigger and reshaping the hammer to speed firing this way. It was called a slip hammer gun. Judge Martin, a friend of Elmer Keith did this.

[TangoLimaSierra]

He pulled the trigger before he cocked the gun. When he cocked the hammer he thought it would be held by the sear. Let go of the hammer and the gun fires.

I thought the same but I think his thumb might have slipped off the hammer. Either way, he pulled the trigger.