Leonard M. Wright Jr., Flutter, Skitter and Skim (formerlypublished as Fishing the Fly as a Living Insect), 1972. Chapter 3, TheSudden Inch, pg. 38, "And now, for the first time, some of those bafflingexperiences from my earlier fishing days began to make sense. Those times whentrout took only at the end of the float, just as the fly started to drag, wereexplained to a certain degree by the effectiveness of the twitch."

Fran Betters: Fly Fishing and Fly-tying Patter Guide, 1986,pg 72. Quoting Bill Phillips: "Now the real value of the Usual is that ifyou do not hit the right feeding lane, you can skitter the fly to the rightfeeding lane without a drag effect. When the fly is in the right lane, feed theline from the reel until it passes where the two currents converge. Generallythe tautness and weight of the line will cause the Usual to go under thesurface, and then I return it slowly as one would a nymph. I don't worry aboutdrag; I use it."

I think it is a good idea to move a fly/nymph/bug like a struggling natural, and so do many before us.