they work by picking up on singnals the bounce around from the radar gun and can pick them up from a few hundred meters away, a mate in tas had one, and they worked realy well, but the down side of them is that they also pick up the signal from automatic doors on buildings.
not sure how they would go with the laser gun that you mentioned viccop, becouse as you said they are only active for a few secounds and they need to pick up the singal as it bounces around, i have a friend in the airforce who is a communications engineer, and he works with a lot of radars and studyed it a uni so ill see what he knows or says about them