Really? What is it?

You grip the phone book so your thumbs are at the middle of edge facing away from you. Use your thumbs to bend the book into a V shape while bending the sides down with your fingers to create space between pages and helps tear them. Once you've torn them enough, it's a lot easier to tear the rest of the book in half

Or you put the book in the oven. Drying out the paper like that makes the pages more brittle and easier to tear.