Valvetronic Variable lift (VVL) is a patented System of BMW used first in its 2001 3-series and is now being impregnated into the 2011 5-Series. Nissan has a similar system in its Skyline Coupe (Infinity G37). What this system does is vary the volume of fuel entering into each cylinder by varying the lift of the inlet valve. It does this continuously with increase and decrease in RPM.
The system adds two main parts to the cylinder head cam rocker arm assembly. The first is an electric motor which gets its input from the throttle position sensor. The second is an intermediate rocker-arm which is turned by the electric motor and transmits its output to the rocker arm.
When the driver calls for more power the electric motor turns the intermediate rocker arm and this causes the valve to open deeper. This helps fuel efficiency since in a conventional engine no matter at what RPM the engine runs the valve opening is constant, which means even at low RPM the inlet valve is a bit more open than needed.
Pictorial representation:
