Probably the same way it works with forward looking radar mounted in a cruiser. It's hooked up to the ECU, knows the vehicle's speed, and does the math. The cop that pulled me over was going west on 401 and i was going east...i got a blip of Ka band radar, nailed the brakes, then the cop pulled into a u-turn and pulled me over. I'd say he was BS'ing but I was doing damn close to 140...I called some lawyer in Toronto and they're pretty sure they can get it thrown out :dunno: