the link kind of explains what it does, I had this done on my car end of last week, and by strapping the reflectors onto the wheels they roll the car backwards and forwards to measure more angles to be able to work out the tracking of the rear wheel and therefore how to setup the tow etc of the front wheel. You should have a look at the posts on here by wheels in motion he talks about the topic and even more advanced techniques.
One thing to bear in mind if the tracking is out and the steering wheel is currently straight then the adjustments might leave it off center, as mine now is (as it was re-centered rather than re-centering the front wheels) so now needs re-centering.
