Haven't owned one, but have worked on and sold a few. The front hub generates power for the system, and also tells it how fast the bike is going. If you want to change the speed it decides to shift at, there should be a little dial on the main unit. They seem to be quite reliable, though getting spare parts for one would likely involving finding a donor bike to strip parts off of. If the electrical system does fail entirely, it should be possible to rig up a manual shifter to control the hub.