All I know is that a Ushiro Mawashi from someone who is a higher grade than me looks like an ordinary Mawashi to start with, so I tried to block Shuto, missed the block and then as the guys heel came back hard for the Ushiro bit it caught the tips of my fingers end on - just like I'd rammed my extended fingers into a wall, the whole dojo heard the crack
Could've been your nephew I was sparring with Ant...he was a black belt, but he wasn't very old - probably 15 or so. Still, quite tall for his age and a lot, lot faster than an old giffer like me....beaten up by a kid...oh, the shame.