Norrell uses his basin trick to spy on Childermass having sex with another character of your choosing. Perhaps he does it on purpose because he is a perv, or maybe he actually intended to make sure Childermass was doing whatever task he set him to, and found that in fact he wasn't but not in a manner he ever expected - and then can't stop watching. Does Childermass suspect? Maybe he knew Norrell was planning to spy on him and decided to put on a show.