Was not suggesting spinning backwards to slow down. Was explaining that the speed is controled by bringing the control arms back towards the operator.
We have a great Dane zero turn commercial mower. I mow some very steep hills and I have complete control of the machine for if I did not, at the bottom, I would get some very nice flying lessons! I creep down the slopes. I have a trail that lets me use a gradual climb to loop around and come back to the slope for a second pass. Cannot climb the slope. The wheels will break and spin every time trying to back up the hill.
I think the OP should practice on some gentle slopes and then move to more aggressive slopes once the control is mastered.
That said, if the hills are such that the machine cannot hold itself back, then another mower is needed.. or perhaps better said would be another mowing technique is needed/recommended.