The STAC Zero Powermeter works effectively on Newton's laws: every action has an equal and opposite reaction.
The Zero magnet array applies a force to your wheel, slowing it down. But because of Newton's laws, this produces an opposing force on the magnet array. The entire resistance unit is mounted on a strain gauge, which measures this force.
Atop the power pod is a reed switch that is watching for the spoke magnet to go by. When it does, it lets the reed switch determine your wheel speed.
Force in newtons (N) multiplied by speed in meters per second (m/s) results in power in watts (W), which is what all our cycling friends are interested in.
Does it need to be calibrated due to different wheel constructions or magnet distances?
It doesn't! Because differing wheels or magnet distances will result in more or less resistance created by the trainer, they also result in exactly as much difference in the force felt by the strain gauge/load cell.