Open up Track IR, highlight your profile, click on edit, hot keys, assign a button/key to pause, center (and precision if you want - tho I dont use that particular one as it slows trackir down too much for me). In game, if you need to stop it, hit pause and then center to ensure you are looking square thru the sites. Once you need it again, hit your pause button and your away.
hth,
Wurzel