Hanako needs to be summoned. In order to call Hanako, you need to do the polite thing and knock three times on her stall door. This is usually accompanied by calling out, "Are you there, Hanako-san?"
If you are greeted with a reply, "Yes, I'm here!", apart from pissing your pants in terror, you can push open the stall door to reveal Hanako. Said to be a little girl with bobbed black hair and a red skirt, the outcome of your courage (or dumbfounding retardation) differs: Hanako-san will vanish or, for the more shit out of luck (in every sense of the word), you will be pulled into the toilet and killed.
If you knock on her stall and receive a reply (and assuming you don't immediately break the laws of physics during your escape), you still have the opportunity to walk away if you do not open the door. If, however, you insist on cornering little girls in toilet stalls, you may have just enough time after seeing Hanako to make a break for the exit and escape.
