What is it?
Green Mario 64’s invisible staircase (acts like a slope) is an interesting feature (may be a glitch). When Mario walks off into OoB (not above a floor), he walks on a temporary slope that infinitely extends downwards that you can walk on to go back to a floor. When on an invisible staircase, you can walk infinitely downwards.
In the ZBC, it is used to save a Z press.