The ones you need are called an LRC100 or the newer one!!! They jam for 5 seconds whilst sounding an alarm....

The idea is that once you hear the alarm you safely brake to below the the speed limit (within 5 seconds!!) and then the unit stops jamming after 5 the 5 secs and lets the copper get a reading!!

With recent things I've heard though, if you have one of these and you get zapped, the best thing to do is whip off the jammer when you get home, for a couple of weeks......

The people who have been done with these on were blatently taking the piss i.e. speeding through traps some 16 times.... By which time the cops got a bit suspicious and decided to have a look at the motor, and found the jammer!!