POP is simply a super fast form of instant on radar. In POP mode the radar's computer triggers the radar on and off extremely fast. The intention of POP mode is to get a reading without radar detectors detecting them.
Radar detectors are like radio scanners, they scan through all the radar bands that police radar uses and alert when they see a signal. POP is so fast that the detector either doesn't see it, or the detector thinks it's a spurious signal and ignores it.
POP mode is only available on a few radar guns which are all made by MPH. MPH recommends that POP mode only be used to get a preliminary speed reading. After the preliminary POP reading the operator is supposed to switch to normal radar mode to get a tracking history speed reading in order to issue a citation, which is mandated in radar operator training.
There are several reasons why the factory default is POP off.
1) When you enable POP mode the Passport
not only scans the normal radar frequencies but it also has to keep going back and looking at the frequencies which POP transmits on. Because it has to keep dwelling on the POP frequencies normal radar alerts can be delayed.
2) POP false alerts. Because there isn't any time for the Passport to confirm whether it is receiving a true POP signal or another non-threat signal, the Passport will alert to any signal it receives on POP frequencies as a POP alert. If you enable POP detection then expect to regularly receive POP "false" alerts. These are mainly from the strong emissions from other cheap radar detectors (eg Cobras).
3) POP is very rarely used today. The guns that have POP are expensive to purchase. Add in the fact that the operator is not supposed to use POP alone to issue citations and the cost benefit just isn't there.
I do not recommend using POP mode, the performance hit and the extra annoyances are just not worth it for the extremely slim chance that you will ever be shot with it.