He pulled the hammer back with his thumb and let go. And the gun just went off.
If you ever saw a western were the shooter "fans the hammer" to rapid fire.
That is basically what he did. He did not pull the hammer all the way back to the locked position. You let go of the hammer without it locked, it FIRES!
True, an old single shot, break open 4.10 shotgun with a hammer we have does that, I was wondering and tested it a few years back.