Acceleration with slope

I would love to see and leearn how to use slope as a accelerator, my wish is to use that slope to go faster if the slope is negative and slower when it will be positive, like ski, surf, snowboard.
if one of you have i actually follow a VR snowboard tutorials, but the linetrace is very strange, but the technique used is perfect to me, no physic used, only math, as i understand.