This is the Speed Sensor 2 and Cadence Sensor 2 supplied together, which is how most riders end up using them. One fits the wheel hub and reports speed and distance; the other straps to the crank arm and reports pedaling cadence.
Neither uses magnets, so there is nothing to align and nothing to knock out of position, and both move between bikes without tools. The speed sensor also records on its own and uploads later, so rides taken without a head unit are still captured.
Together they fill the two gaps a GPS computer leaves: accurate speed where GPS is unavailable, and cadence, which cannot be inferred at all.
