In a sense a thermin is like the traffic loop detectors, but a thermin is multiple loops with better electronics and sensitivity. I like Ms Alice Malloy's bra thermin - sounds a lot better then a guy beating his chest. But a moving magnet induces a voltage spike as its moving magnetic field cuts across the stationary coil in the road. Thus while a loop of metal or flat metal is detected the magnet increases the magnetic field of the coil of seat tube and rear stays. One could lay their bike down in the middle of a loop to get it to detect the bike better. Also the loops of the wheels would aid an induction change in the loop even if non-magnetic but current carrying aluminum. But the magnet aids the detection. Ultimately some engineer may have set detection to low to detect many bikes so as not to detect a truck in another lane.