by reading the user manual I found out that the cadence is coming from the Zephyr HxM belt. my guess is the cadence comes from the analysis of the values of the 3D accelerometer incorporated in that belt.
maybe Tom or Robert can elaborate on this. I can imagine that, for running, by analyzing the vertical acceleration spike, one can deduce the cadence of your foot step/stroke. But for cycling I don't know if the zephyr can also measure some kind of cadence (maybe there is also a typical horizontal spike in the acceleration diagram of a cyclist, from which one can deduce the pedal stroke cadence ??).