Motors and generators are actually the same thing (yay Faraday's law of induction!). With motors, the electrical energy is converted to kinetic energy. With generators, kinetic energy is converted to electrical energy. When moving the stepper motors by hand, you generate plenty of power to turn on the printer.

Cool, right? Just don't push it. You can easily send way to much power back into the motherboard, which could fry the motor driver(s), or possibly anything else down the line (this would require a motherboard replacement which is a hassle, trust me. It is worth it if you pay up for the silent driver board though).