RPM sensors are normally installed near a phonic wheel, also called flywheel (typically, a sprocket wheel or a ferromagnetic cam).
They detect the variations of the magnetic flow produced by the alternation between the presence and absence of the tooth or cam of the flywheel which generate an electrical signal.

This signal is sent to the engine control unit whereby, depending on the phonic wheel where the sensors are installed, the detected signal indicates:

• The revolving speed of the drive shaft (Crankshaft Rotational-speed
Sensors)
• The top dead center by detecting the position of the drive shaft
(Crankshaft Position Sensors)
• The ignition sequence/timing by detecting the position of the
camshaft (Camshaft Position Sensors)