I think my problem with it and why I don't understand it, is that the board with the minutes and degrees on it, doesn't fit to the other wheel, so how can it know where the toe is? My friends kit fits to both wheels, so there is a physical connection to each wheel.
In effect, what you do is set up the laser and scale assembly, relative to the left hand front wheel (i.e. the wheel furthest away from it). You stick the mirror up against the rim, making sure both plastic arms are touching the rim. This means that the mirror (assuming the aluminium pole it's mounted on, isn't bent), is now parallel to that wheel.
You then fiddle with the laser until the reflection of the dot is on the "zero" line. At this point, the laser's beam is at exactly 90 degrees to the wheel furthest from it.
You then take the mirror assembly round to the right hand side of the car, nearest the laser, and set it up against the right hand wheel, so that the mirror is now parallel to the right hand wheel BUT YOU DON'T TOUCH THE LASER.
If both wheels were absolutely parallel to each other, the laser would read zero. Whatever it does read, is the total toe (in or out) of both front wheels. You'd then adjust each track rod equally until you get the toe setting you need.
One downside (apart from setup time, which is longer and messier than with "twin" gauges), is that a lot of manufacturers quote toe settings in millimetres at the wheel rim, rather than in degrees and minutes, so you need to do a bit of trigonometry to convert from one to the other.
There's a video on their website, which might help explain it better:
Download the Trackace manual to save or print & visit the Trackace help site for help in adjusting your own tracking at home.
www.trackace.co.uk