Watchman speed cameras are similar to the popular Gatso cameras in that they emit radar signals to detect speeding vehicles. They differ in that they have two types of cameras installed, one to get an overview of the road ahead, the other to capture the number plates of vehicles that are late (Number Plate Recognition System). 
Watchman emits a constant stream of radar, and calculates speed as a vehicle passes the beam and triggers a camera if the vehicle is speeding. Because Watchman has "rear-facing eyes" that slow down as you get closer to the camera, your license will not be saved as your speed has already been recorded, unlike traditional "Gatso Style Zones".
Don't get caught by speed cameras, buy a Snooper Speed Trap Detector.