I'm going to explain what I was taught by a judo instructor. I don't know if he designed this or someone else.
His method was to not go for retention but to disable the person attempting to take your gun. Empty your gun and holster it. Have someone, "Bob" here, grab and hang on to your gun. I'm explaining right hand strong side carry since I'm right handed. Get your right arm behind Bob's arm with the bend of your arm below Bob's elbow. Swing your right hand up smartly and lift. This traps Bob's arm and hand against your side. As demonstrated to us even if Bob got the gun out he could only shoot the floor not you. I consider the next step shown optional.
The instructor would take his left hand, thumb extender, and using the Bob's nose as a guide, shove his left hand thumb into the left side of Bob's right eye, getting his thumb behind the eye and pulling forward to remove the eye.
When you try this be very careful because it seems like you could break your Bob's arm very easily. If it were a BG than a broked arm is a plus. The eye removal is still optional.
I am not LE or Judo trained. I was just so impressed with the demo it has a permenate spot in my brain.