A pocket supports the flesh on your tip, as it follows the shape of your tip. Both sides in case of a mono, single in case of two finger pocket.. stating the obvious I realise. With a rung your flesh isn't supported and 'squishes' in technical terms out sideways. Quickly becomes painful and bruises quicker. Try hanging a rail as middle finger hangs, like you would a mono pocket, you'll soon be stopping no matter how 'ard your tips are.