Basically:
This mod allows your inputs to register in between visual frames, which drastically increases input precision on low framerates like 60FPS.
There has been some debate as to whether or not this is cheating. The official demonlist has allowed it and a lot of skilled players are ok with it, while other people (including RobTop) say it is a cheat.
Do you think it is a cheat or not?
Nah. I don’t see why someone’s game performance should be tied to what hardware they can afford (IE a high refresh rate monitor), instead of purely their skill level.