Pulls timing as the boost increases. There should be a control nob somewhere that dials from I think 0 to 2? Don't remember for sure and my cars covered so I can't go look.
From my understanding (might be wrong) the box pulls how many ever degrees of timing you have dialed in per pound of boost. So if your dial is at 1, it will pull 1 degree for every pound. I keep mine set at about 1.5 if I recall correctly, but on a day with nice cold air I can drop it down to .5 or so and still not ping.
As engine rpm increases, so does the amount of timing in degrees. This is how it is measured. On a motor with a supercharger, This device allows you to run a little more timing at lower rpm's w/o the threat of detonation in the upper rpm's. Example: if you have your BTM set at 1, than for every pound of boost the engine is under, the BTM retards or subtracts 1 degree of timing. So if at 6000 rpm's your total advanced timing was at 35 degrees, the BTM would pull the timing back to 25 degrees. Hope this helps out.