Muggles' Guide to Harry Potter/Magic/Diffindo

Overview
Diffindo is a destructive spell. It can be used to make seams rip and books separate from their bindings; its effects can be reversed by the Reparo charm.

Extended Description
Harry first uses this spell in Harry Potter and the Goblet of Fire to separate Cedric Diggory from a clutch of his friends, by causing the seam on his bag to split. Cedric has to stop to collect his books, which gives Harry a chance to speak to him privately.

In Harry Potter and the Half-Blood Prince, Harry uses this spell to keep his copy of the Half-Blood Prince's Potions book. When his own copy of the book arrives from Flourish and Blotts, Harry uses the Diffindo charm to separate the covers from the text of both his new book and the Prince's book, swaps covers, and then uses Reparo to repair both. In this way he can return a book to Professor Slughorn that looks like the one he borrowed, but keep the incredibly useful marginal notes of the Prince.

Analysis
Hermione is, of course, scandalized at any use of magic for destructive purposes; in particular, when this spell is used on a book, she is horrified.