Nearly every RADAR unit on the market today has a "hold" feature which allows the officer to prevent the RADAR beam from leaving the unit until he/she wants to check the speed of a vehicle. It is very common for the beam to be released, a speed obtained, the speed locked in, and the beam held again all within a matter of a second or two. That is much faster than you can react to your radar detector, brake, and get your car slowed down enough to avoid being stopped. In fact, the majority of detectors cannot even be activated in that short time. So, if the officer is using the unit to its full potential, you'll get nailed every time.
This was taken off a web site i just found