If you are driving in the same direction mode, the faster / slower button tells the radar what the target vehicle is doing...going faster or slower that the patrol vehicle. Basically, it changes the mathematical formula to calculate the speed of a motor vehicle...There is more to it, but that is basically it.
The whole 75% thing is that the patrol vehicle has to be going with in 75% of the target vehicle speed in order to get a reading. So, if the target vehicle is going 175Km/H, and the patrol vehicle is going 99Km/H, the radar will not register a target speed. If the patrol vehicle is going 100Km/H a speed on the target vehicle will be obtained...Again, for same direction mode only.