It's possible, but I'll have to look up what supinated means to tell you what the arms doing, Grimmett bowled it, Bradman noted it as Grimmetts mystery ball and we revived it on this forum.
Yep - supinated sounds likely.
So its basically using the same arm angle and same resulting spin as an offspinner's topspinner (if you follow me). Are there any particular advantages from getting the thumb involved, would you say?