In chin ups, the arms are pulled downward and backwards — a movement called shoulder extension. (fitnessandpower.com)
In contrast, Ackland et al. (2008) showed that the posterior deltoid displayed a peak shoulder extension moment arm length of -33.0 mm at 30 degrees of flexion (arm pointing down), indicating that it likely acts as a primary shoulder extensor, second to the teres major while the arm is close to the body. (strengthandconditioningresearch.com)
The lats are best trained with shoulder extension movements that don't allow the biceps to get involved and steal the spotlight, and movements that enable a constant tension throughout the range of motion. (fitnessandpower.com)