The engine speed is registered by the crankshaft speed sensor.
This inductive sensor supplies a sine-wave signal with speed dependent amplitude and frequency.
In order to discharge speed-synchronous functions, the DDE control unit uses this signal to generate a number of pulses proportional to the cylinders (e.g. for advance/main injection).
The speed-monitoring facility can also detect engine overspeed.
Overspeed is counteracted by the gradual fadeout of the speed-synchronous functions.