Here is a video: Skip to 40 Sec to see it in action
By changing the grass options from Non to High you change the terrain at high distance. In Video you can see that all that happens is the terrain changes but grass does not become visible.
With no grass, the ridge of the mountain drops and anything on top seems to be flowing around. This makes it extremly easy to spot and shoot enamies.
Just checked and this also causes enamies to dissapear under the ground or clip through it.