Steve, can you explain more? I remember reading Marinaro's book some years ago that the hackle would make the indentation on the water film as the legs of the real insect would do. I don't have the book here with me now but will take a look at that passage again tonight.
See the picture from Castwell work here in FAOL "flies only"
http://www.flyanglersonline.com/feat...ctionfive2.jpg