What is a rear naked choke?

Answer 1

A rear naked choke is a submission hold which restricts the arteries in your neck to halt the flow of blood to the head, resulting in unconsciousness. The blood flow resumes once the hold is released.

The hold is performed by curling one arm around the neck, so the bicep and forearm are positioned over the arteries, and using the other arm to both secure the first arm (the hand of which grips your bicep) and to push the opponents head forwards into the hold.