Yep, what Chalenge said - you can sit further forward when centering, so it gives you more backview, but I just lean around - make sure your trackir is set to above normal in the process list in task manager (bring up task manager, right click, set priority, abovenormal) and in the settings for AH, look for the relative to plane check box, and untick it (once done, if your looking back and lean left, you actually go left rather than right - as its not relative to the plane any more).
HTH,
Wurzel