1. You have the guy’s hand in your hand for the handshake already.
2. You know which way an elbow bends.
3. Grip his hand, pull his arm straight
4. Rotate his hand outward forcing his elbow to point upward.
5. Use your off hand to apply force on his upturned elbow in the direction that it does not bend, and guide his face toward the floor as his body gives way in an involuntary reaction to avoid the arm break.
Hold him as long as you want.
It’s a slight variation of this simple, 7 second video (this isn’t a cross body grip like a handshake, but the mechanics are the same. Note, you have to control the wrist and elbow for it to work)