Instant-On is a term used to describe a particular method of operating radar that can be used to defeat radar detectors:
The officer puts the radar gun in "standby" or "hold" mode where it is on and ready, but not transmitting any radar. When he visually identifies a vehicle that he suspects is speeding, he switches the radar unit to "transmit" mode and targets the vehicle to obtain a speed-reading. If he chooses, he can then switch the unit back to "standby" or "hold".
Here are a couple of ways to defend against instant-on:
1. Choose a very sensitive radar detector, so that when traffic ahead is targeted with instant-on, you'll get a warning alert before it is "your turn".
2. Never stand out in a crowd, or you'll be the one he chooses to target. Try to follow behind someone else who is going the same speed as you are, maybe 1/4 mile behind him. That way, if you encounter radar, he'll get hit with instant-on, and you'll get an alert on your detector telling you to slow down.