So here is the hybrid stance:
As has already been pointed out, FED wants to eliminate this stance on the grounds that it meets the definition of neither legal stance.
It is not a windup, since the free foot is not on or behind a line through the front of the rubber.
It is not a set, since the pivot is not completely in contact with the rubber (the toes hanging off the front are illegal). Yes, the rule is written poorly, but no more poorly than OBR or NCAA.
IMHO, a pitcher gets more advantage from being allowed to windup from this stance than if he sets. THAT (the hybrid windup) I will ball/balk every time.
I don't see that he gets much advantage from using this stance as his set. Provided he complies with the rest of the rules concerning pitching from the set, having toes hanging off is no big deal.