Variable valve timing units controlled by the engine ECU.

Yes but how?

The Valves are operated by cams, on a camshaft, that is linked mechanically to the crankshaft via a belt or chain - so how can electronics alter the valve timing as they are set by the crankshaft and mechanical linkages

The camshafts have special pulleys that are operated hydraulically with engine oil being fed under normal oil pressure to different chambers within these pulleys, it is usually done with electrically operated oil valves. You can see the pulleys in this video of the Ford Ecoboost engine. www.youtube.com/watch?v=IFj5UIZx2cE