Here's a great video from Tightline Productions with our friend Tim Bartholomew showing how he ties a Hackle Stacker Hendrickson. Its a technique that can be used to tie any mayfly adult pattern that, as Tim says, creates a fly that may be taken for a mayfly dun, cripple or spinner. Its a technique that will take some practice to get the hang of, but once learned, will add another pattern type to your dry fly box. I like them for the smaller mayflies - Blue-winged Olives and Sulphurs.
Sharpen your hooks!