Looks like robber's left arm totally dislocated from shoulder socket leow.
Siccessful arm lock means the subduer is using the muscles from 2 arms, 2 hands, 2 legs and torso plus bodyweight against the opponent's elbow/shoulder joint muscles of a single arm. The difference in leverage is huge.
Also, damn easy to break the opponent's arm. Just need to use a bit of force to pop the elbow joint sideways. That joint is naturally strong against attempts to bends it backward, but weak when being forced out of the socket sideways.