If you measure the torque on a chart, it's going to be a repeating cycle twice per revolution of the rear hub, one for the left foot, one for the right foot, by extrapolating that, they can give you your cadence. Of course if you were lance armstrong and can apply 100% the same amount of power through your full stroke, then it probably won't be able to tell.

Kinda like using a really sensitive seismograph when your neighbors are doing it next door. You don't have to watch or listen to them moaning, you can probably tell when you see a certain pattern on the chart repeating itself at a certain rate.