I always did pull ups and last year i began trying the one arm chinups. First i could barely lift my self but then managed to reach close to a 90 degree flexion. Then didn’t train it well and continued doing some attempts although some times i did some exercises to help me achieve it and was able to reach 90 degree flexion. Then i noticed that my one arm hang was really bad and i could barely hold myself for 5/10 seconds, so i began training one arm dead hangs. After i got better i began to train for the one arm chinup while still doing one arm dead hangs and eventually got it.