How does the author use the "cliffhanger" to end Chapter Four?

From the time the children return to class in Chapter Four, which is accompanied by the return of the humming and glowing, Conner's internal need to protect his sister from the teacher causes an instantaneous and ill-conceived idea to throw a book at the teacher's head. This incident allows Alex the ability to leave the classroom before Conner, as the teacher makes out detention slips, but Conner has an epiphany that forces him to leave the teacher unexpectedly. The notion that Alex will jump into the book herself is predictable, and Conner races home before it can come to fruition. This triggers the event that not only closes this section on a cliffhanger, but provides the most important event in the narrative as Conner trips and both children fall into the book.