Performed specifically, and heavy enough to be useful, the arm bar stretch is an excellent mobility drill, positional assessment assistant, and static strength builder.
This movement begins, for us, with an organized kettlebell floor press, and does not progress until the weight is stopped and secured in a lock-out.
The thumb drives back, and “out” throughout each transition of the movement, allowing the arm to stay 100% locked in place, even during the roll-over and return to your back; Do not allow the thumb to drop inward- just as you would not in a heavy Turkish Get-up.
Proof of positional maintenance here goes a long way to ensuring stability in the bottom position of corresponding lifts such as Turkish Get-up and Windmill.
This should not be casual, and there are more relevant details than meet the eye- find them, wp-contently them.