It's not in the manual, at least in my quick skim of it this morning.
I'm the lead author of Kindle Paperwhite for Dummies (also Kindle Touch for Dummies). I started a thread on Kindleboards asking for tips and hints for the book and one of our members posted this tip, for which I am very grateful. I was starting to panic about doing the book without screenshots! The member who posted found it at Len Edgerly's Kindle Chronicles website. I don't know if Len figured it out on his own or someone told him.
And that's the whole truth.
MW