One-arm Pull-up
The one-arm pull-up (OAPU) is the hardest single-bar pulling skill in calisthenics. From a one-arm hang you pull until the chin clears the bar using exclusively one arm. An absolute elite move few people worldwide perform cleanly.
Requirements
15+ strict pull-ups, solid OAPU negatives (5-8 sec), archer pull-ups with minimal assist-arm support, and years of specific unilateral work.
Execution
Grip the bar with one hand, pronated or supinated (supination is usually slightly easier at first). Free arm at the side or behind the back. From a full hang, pull with max force until the chin clears the bar. Control the descent. Slight rotation is inevitable but should be minimized.
Free arm
Some athletes grip the working wrist with the free hand for light assistance (assisted OAPU). Valid in progression but does not count as a full OAPU.
Note
A clean OAPU—minimal rotation, full range—is one of the greatest milestones in calisthenics.