On the Boeing 737-300/400/500, initially a great idea, the VSCF has proved to unreliable. To the point where some operaters have removed them.
The Constant Speed Drive is basically a Hydraulic motor and a pump in one, this is to allow the varying input speeds from the engine during it's operation to give an almost constant output speed to the generator to supply the 400Hz required for the aircraft systems.
The Variable Speed Constant Frequency is a self contained DC generator with an integral electronic package which converts the power to AC and a stabilised 400Hz, during all phases of engine operation.
http://www.esw-wedel.de/dt/download/vscf.pdf