Also, keep in mind FTM and TPMS are two different things in the BMW world.
TPMS was mandated in the US after the Ford Exploder incident. It utilizes sensors in the wheel itself.
FTM uses your wheel speed sensors to detect a flat tire, it won't actually be triggered until about a 6psi difference from the initialization pouint. It's pretty simple actually, a tire with less air in it has a smaller diameter, thus it spins faster. I know the Canadian spec BMW's use FTM, not sure if the US car's switch to FTM, or they sttill use the TPMS system. I haven't seen a US spec car in a while.