As others have said, shoulder stability is a prerequisite for one arm hangs.
A good test is to check whether you're able to hang one armed from a bar, controlling the shoulders and engaging them as if you were to do a one armer. It's tricky to "feel" that move.
A good practice is to hang two armed from a bar and try to pull up just by retracting the scapulae (that, by the way, should be always done when doing a pull up, instead of flexing at the elbows). The upwards movement will be quite short but you'll feel it in your upper back, especially lower traps and rhomboids, but lats also.
I hope this helps a bit more than the video.