Metalcraft Racing

I have a question, Im already regretting asking, hoping I can only get legitimate answers, not dumb ass opinions or comments, here we go, I was looking at a late model Harley with abs, the sensors are mounted in the axle spacer, nowhere near the rotor bolts, what is it counting to establish wheel speed, Speedos count gear teeth, i understand the concept, wondering what it's sensing? I inspected the rim pretty closely no signs of a trigger of some sort, legitimate responses appreciated, and yes I know the pic doesn't have the sensor, it just for reference,

Vintage fuel bearing support, one piece at a time,. www.metalcraftracing.com