I call it mountain clip. Basically if you align your horse at a specific angle on the edge of a mountain, dismounting your horse at that angle will clip you through the invisible wall and you can run around on mountains and jump outside the map.

Takes a bit of practice at first but you'll eventually get the hang of it with visual cues and whatnot.

I've found this can save a lot of time on Wu Zhang Plains and potentially Hu Lao Gate.

Tested on DW4:XL