What is "The Word Shaker"?

"The Word Shaker" is a book that Max wrote for Liesel before he was sent away from the Hubermann household. In the book, a young girl walks around the neighborhood shaking trees for Hitler. The young girl climbs up a tree that is about to be cut down, and is later joined by a young Jewish man. The two sit in the tree, watching it bend, confident that it will never break.