"They stuff people's heads down the toilet the first day at Stonewall," he told Harry.
"Want to come upstairs and practice?"
"No, thanks," said Harry. "The poor toilet's never had anything as horrible as your head down it –– it might be sick." Then he ran, before Dudley could work out what he'd said.
(Harry Potter and the Sorcerer's Stone)
I am not sure if it refers to the toilet or has some other role in the sentence. What is its role?