A few times during the film, we hear Bill muttering, "He thrusts his fists against the post." Pennywise even murmurs it just before his final blows from the Losers Club. In the books, this is actually an important phrase for Bill and a huge part of defeating Pennywise.
The full phrase is, "He thrusts his fists against the post, and still insists he sees the ghost." It's supposed to rid Bill of his stutter. If you're familiar with the book, you'll also know that Bill is the one who ultimately defeats It. Using something called the "Ritual of Chüd," Bill basically challenges Pennywise to a battle of wits in a different dimension. This little tongue-twister is part of what he uses to win.
