Back control is the most dominant position in BJJ. You are chest-to-back with the opponent, who cannot see your hands. A seatbelt grip controls their upper body; hooks behind their thighs or a body triangle controls the hips. The rear naked choke is the primary finish in no-gi; collar chokes add options in the gi.
The attacking player maintains connection and works to secure the neck. The defending player protects the throat, fights the hands, clears hooks, and tries to put a shoulder to the mat to turn into a safer position.
Back control scores four points in IBJJF competition and is widely considered the hardest position to escape cleanly.
For full coverage, see Back Control.