To perform this, simply turn into Forge mode on any map, then while still in Forge mode, fly into a kill barrier, a kill ball, or have another player kill you. You will respawn as a monitor, albeit with the standard SPARTAN-III HUD - and you keep your SPARTAN-IIIs health, and Forge controls are still the same. To remove the effects of this glitch, simply go to player mode, then back into Forge mode to find the HUD back to the default state. Sometimes on some TV's but not at all TV's, a rare glitch will happen that makes you only can see half of the map.
In some copies of Reach, this glitched HUD is the default for the monitor, and the only way to access the default is to kill yourself in monitor mode.