Its basic electronics and you should know hehe
A fan motor is a magnet moving inside a coil of wire, if you pass current through the wire it make the magnet move, if you move the magnet it creates current as you have discovered,
Never do that if your fan is connected to the Mobo by the way as you can damage the Mobo.
Hehe, you opened this as a discussion, not a question I reckon
the B.a. button is normally on every answer post, button on the top right of the reply saying 'select as best answer'
if thats not there, you have opened this as something other than a 'question with answer'
No probs though mate