With TrackIR, if you look a bit to your left or right (ie, looking left to 310 degrees if straight ahead is 0 degrees) and move your head in the direction you're looking, it doesn't zoom in.

To zoom in when looking to 310 degrees, you must keep your head in that direction, whilst moving it sideways 90 degrees. To zoom out, you must move your head sideways 180 degrees.

This is mirrored on the right hand side.

EDIT:

It seems to be that zoom is based around forwards relative to your in-game body, not your head. So if your in-game head was facing to 180 degrees, but your in-game body was at 0 degrees, you'd have to move your real-life head *away* from the direction you were looking in-game (ie, towards the way your in-game body was facing - 0 degrees in-game) to zoom. This means that you'd have to move your real life head *backwards* to zoom in.

This feels awkward and unnatural.