I have this detector and it works very well, no matter where the cop is located, it picks up radar signals travelling through the air. As with any detector the cops radar has to be on. The cop can be driving or sitting idle with it on. Where I live the local cops tend not to drive around the city with their radar on but the state troopers leave it on almost all the time. Doesn't guarantee you wont get a ticket but has certainly saved me from a couple.
Yes, this is how all radar detectors work. If a cop is very careful to keep it off until they target your vehicle specifically, it's very difficult to avoid a ticket. When they "work" you are detecting radar from cops targeting other people's vehicles.