What is a valvetronic on a mini cooper?

Valvetronic is BMW's name for its variable valve lift technology. It can vary the amount of lift at the valve to suet the needs of the motor. It consists of an eccentric shaft that is moved by an electric motor, increasing/decreasing the amount of lift at the valve. There are YouTube videos and in-depth descriptions on how it works.

FYI, BMWs also use it to control engine speed, the Throttle body on BMW engines is there as a safety feature required by law and is actually open 100% when the engine is running. This reduces pumping losses, increasing HP and MPG.