Sometimes called "pulse radar" or "RF hold". Refers to a feature of some modern police radar speed measuring devices to sit in a warmed-up state without emitting any radar pulses. When a car comes into range, the police officer activates the radar and a speed measurement is taken almost instantaneously.

Instant-on radar is infuriating for speeders, because radar detectors cannot detect the radar gun until it is actually emitting radar waves. Luckily, the pulses caused by instant-on radar being used on traffic ahead of you can bounce back to your detector in time to give you ample warning; however, if you're the only car on the road, you're out of luck.