Thumb behind the slide is not a good idea when your finger is on the trigger. Ouch!
Maybe that's the smart gun that recognizes his hand/fingerprints, and so it would sense his thumb on the slide, and be inoperable. That would explain finger on the trigger...
THIS IS HOW WE DON'T HOLD 2 HAND GUNS,THANK U
judging by his stance i would say that this is a scene were he is pretending to surrender and is decocking the sig because it is in his left hand and the decocking lever is on the left side of the hand gun making it hard to reach with your thumb so it would be easier to do it the old fashioned way which is thumb on hammer finger on trigger
....just a movie guys, relax.