Technically, what it is supposed to do is not render any object being covered up by another. Thus saving fillrate and memorybandwith.
As far as 3DFX's HSR (which may or may not be simmilar to what Nvidia is implementing), there are viaribale options of how much HSR you want. Probably how much has to be covered up before it is deamed as what should be unrendered. As such, the more you set it to remove the faster it goes, and the more chance that something will not be rendered that is visiable leaving artifacts.