Hence, it determines the exact position and direction of your head, more than 120 times per second.
You can even look backward while still looking at your screen thanks to a TrackIR configuration called Motion Scaling whereby beyond a certain angle, your moves will be amplified. All is perfectly adjustable in TrackIR thanks to an elaborate setting utility, with predefined profiles for the most popular games, and generic profiles for specific game types (flight sim, car sim, FPS, etc.)
When a game developer builds support for TrackIR directly into the code for their game, we call that game "TrackIR Enhanced".
TrackIR is a new form of input that doesn't affect the mouse, keyboard and joysticks you are already using. Those other inputs are now freed to do the things you'd expect your hands to be doing, completely independent of your new head-tracking view controller.
This whole experience is commonly called a "VR fishtank" method (as opposed to the "VR Goggles" experience you may have seen in movies), because you don't have to wear any sort of headset or hardware. Everything is handled using advanced motion capture techniques.
The point of TrackIR is to make controlling your game's view completely seemless and natural. You will forget you have TrackIR setup at all, because looking at your monitor is like looking through a window into a 3D world.
Customization is the Key :
What makes TrackIR much more than joystick or mouse are the ways TrackIR's software lets you customize how your movement will be processed for each of the 6 Degrees of Freedom.
You can make your left/right rotation very fast, so you can turn 180 degrees in your game by just moving an inch in real life. You can then make your up/down rotation very slow, perhaps because you're in a car and there's nothing of interest to look up or down at.
You Want 6DOF Control, Even For a 2DOF Game :
TrackIR tells your game exactly where your head is, in 3D space. Not all games take full advantage of this input, because there wasn't an easy and cheap way to control your game in 3D space before TrackIR. But since TrackIR knows exactly what you're doing, it will be easier to control the game. For example, leaning to the right won't be mistaken for turning your head. That's the benefit of separately tracking all of the "6 Degrees Of Freedom" in which you can move.