The arched pull ups is a back Calisthenics exercise that consists in performing a pull up while arching your body backwards as mach as you can. The main purpose of this exercise is to control the scapula position along the hole pull up. This can be very helpful when training for regular pull ups as it's a movement that requires exceptional scapula control.
The arched pull ups can be performed using a bar (in prone, neutral o supine grip) or in the rings.
It's important to pull until you feel you can hold the proper scapula retraction and depression. Rounding the back at the top part of the pull will not be considered as a proper pull up. This exercise is so technically difficult and demanding that you shouldn't need the use of extra weights.