Variable valve timing is a simple idea that took time to be implemented due to practical engineering difficulties and cheap fuel prices. The idea is to adjust the opening and closing cycles of the valves to change according to engine speed. When the engine is operating at low RPM, at speeds slow compared to the movement speed of air, the intake and exhaust cycles can have almost no overlap. At high RPM when the air has trouble moving fast enough to keep up with the engine there has to be a large overlap.
The video gives an overview of the ways to achieve variable timing. Watch and learn!