Most of us have an upwards swingpath on our topspin forehands.
Seems likely the same upward swing path is good for the topspin backhand, taken earlier out in front with a closed body stance.
And we don't hit all our shots on the rise.
As for the supination thru the stroke, I think it's an advanced technique. Better to hold the wrist solid throughout the stroke until you can hit it pretty well, then it's up to you to modify and adjust.