So I've heard of it, I don't understand how it works.

Using the arguments below, you can force your graphics card to use a lower shader version to gain better performance at the cost of some visual quality. Of course if the shader version you're trying to force is above, or equal to your card's maximum shader support then the argument will do nothing for performance.

-Use20 Forces the game to run as a Shader 2.0 card. This shows all graphical effects in the best possible visual quality. You will need a recent graphics card with full PS2.0 support such as the Radeon 9500 series or higher, or the FX 5600 series or higher. PS2.0 gives the biggest performance hit but runs Halo exactly as the developers intended it to be seen.

-Use14 Forces the game to run as a Shader 1.4 card. If you have a relatively new graphics card such as those mentioned above, then forcing PS1.4 mode on them will improve performance at the cost of some visual elements, namely:No bumped mirrored surfacesSome video effects are two-passThis doesn't reduce visual quality greatly and can improve performance, particularly on lower end recent cards. Obviously if your card only supports PS1.4 at the most, it will already be running in this mode.

-Use11 Forces the game to run as a Shader 1.1 card. Once again, forcing this lower Pixel Shader mode on cards which support PS1.4 or PS2.0 will improve performance at the cost of the following elements:No bumped mirrored surfacesSome video effects are two-passNo model self-illumination (excluding some specific environmental models)No animated lightmapsFog calculations are triangle based, not pixel basedNo specular lightsThis can significantly improve performance on such cards, but older cards will likely already be running with such effects removed.

-UseFF Forces the game to run as a fixed function card. This means that that regardless of your graphics card's capabilities, the game will run with the most basic effects. Along with the PS1.1 effects disabled above, this results in the removal of the following elements:No shadowsSimple active camouflage effectNo glows or flaresVery basic fog, water and lighting.
This can provide a significant performance improvement, however it is obviously at the cost of visual quality and effects. Older graphics cards (such as GeForce2 series) will likely be running under this mode anyway.

If you really crave a performance improvement use this switch (useff). I recommend that you only experiment with the above switches if you've tried most of the other tweaks. If you have an older or less powerful graphics card, it is already likely to be running in PS1.1 or Fixed Function mode anyway, so these switches are only really useful for those with cards which support PS2.0 or PS1.4 but who require a boost by forcing the card to run a lower mode.

Commonly asked questions;

(1)
Q: How do I apply these to my game?
A: It's a parameter, you can add it to your Halo target line.

(2)
Q: useff makes all HUD and crosshair disappear, what do I do?
A: useff only works in Windows XP and Vista. Certain visual elements will be disabled on Windows 7 etc.