Originally Posted by Clythio

Powertaps work this way too, clocking each 2 torque peaks one pedal turn, and it's pretty good and accurate if you pedal all the time pushing some torque every pedal move, like on a training session interval.
It doesn't have any lag, but it floats a lot out of reality if you soft pedal, if you stop pedaling on a downhill or inside a group, drafting, like we do on a race peloton, which hurts some metrics like average etc, and lay some crazy high spikes on the file. That's the reason I use Powertaps WITH a dedicated cadence sensor (left crank mag attached).
InfoCrank now uses the same principle - torque peaks from one of the cranks, to count crankset revolutions.