✅The movement starts with you just hanging on the bar with your palms facing towards yourself.

No muscle is in tension and arms are let loose.
From those point, just like pull ups, you need to retract your scapulas back and pull youself up with the goal of making your chest touch the bar: that will automatically make it somewhat of a "sway" movement instead of a "vertical" one, as you can see in the picture.
By doing so, you'll unconciously keep your scapulas back at the top, which is exactly what you wanna do when performing chin ups to fully engage your back.

By the way, you don't "have to" bend your legs.
I just felt like drawing it this way.✅

❌On the right the dude pulls himself up without tucking his scapulas back at the beginning.
That completely disengages the back muscles, making it become a bicep dominant exercise.
Plus, all he wants to do is to go up on a straight line, so his shoulders rotate forward, causing nerve & tendon pinching in that area. ❌