Electronic Control Unit (ECU)

I suppose that as you haven't described what sort of electronic control unit you mean the answer is...

Electrons are moving around very fast inside in order to do what ever the electronic control unit was designed to do.

If only all questions were this easy... *sigh*

An electronic circuit works to control whatever the controller is supposed to be controlling.

In a nut shell, an ECU is a micro computer/processor that takes information from a system in the form of current or voltage or open/closed circuits (system is monitored by press/temp transducers, potentiometers, etc.), and generates output commands to various system control mechanisms in order to bring the system outputs into preprogrammed and desirable limits.

you can thnk how do you take a thing by hand? use your eye, brain and then hands.

the elctronic control unit do same way as above. some of them are simple others are complex. but the principles are same.