What's the deal with the hibernate feature?

So I read upon the hybernation feature, and read that it draws a miniscule amount of power to save whatever is running over long periods of time. I put the computer in hibernate to take it apart to clean it, removing the battery, hard drive and DVD drive. I put everything back, started it up and it said resuming windows.

wtf? All power was gone from the hard drive, and the battery wasnt there to power ram or anything, so what realy happens when you put the computer in hibernate?