I'm reading The Fly Fisher's Playbook. In the book the author, Duane Redford talks about nymphing and about vertical drag caused by the different speeds of the water: the surface, the mid-level, and the bottom. To prevent this kind of drag, he suggests using a vertical mend. But after reading his description of the technique, I'm still not sure how the vertical mend is done.

Can anyone describe the technique to me?