Unlike traditional speed or cadence sensors, the RPM sensors do not use a magnet to detect wheel or crank rotations.
Instead, a 3-axis accelerometer is used to determine the orientation of the hub or crank arm and thus your RPM.
Cadence calculations are made with a simple count of those RPM. Speed is calculated using the RPM with wheel circumference data you input.
For information on how to set your wheel circumference, see How to set wheel circumference for a Speed or Speed/Cadence Sensor in the Wahoo or ELEMNT App.