it's a bandpass filter (a narrow one) with a peak gain of near 100 at 15hz. lower frequencies are rolled off to keep the slow movement of sunlight, changing shadows, etc... from triggering it, as well as higher frequency transitions... some engineer figured this response was very close to the optimum response to detect most motion of animals or people walking or running... if you saw the mythbusters episode about defeating motion detectors, the best way is to move very slowly, and now you can see why. a single large object moving faster than what would produce a peak response will still produce a transition within the response of the filter, and likely be detected, but smaller objects in motion that fast may get filtered out.