Diffusion Hackling
Jason Borger, Fly Fish America
Diffusion Hackling is not hackling in the traditional sense -- winding a material like a feather around a hook or a parachute post to produce legs and/or wings. Rather, Diffusion Hackling splays (or diffuses) materials to achieve similar (and in some cases, much better) effects.

Originally, this technique was a European method for "parachuting" deer hair that was shown to my father and me by Charles Jardine. After some experimentation with a wide variety of materials, I termed it "Diffusion Hackling."
Here's how it works: For flies like a caddis adult, for example, tie in a parachute-style post of almost any material at the rear of the thoracic region (the post could be foam, deer hair, chenille, poly yarn, Z-lon, or whatever). Do NOT reinforce the base of the post with thread. This is Post #1 (Figure 1), and it will make the top of the thorax.
Then, tie in another post of fibrous material DIRECTLY in front of the first post (this material will make the "hackle"). By directly, I mean the posts should be right up against each other. This second post is Post #2 (Figure 2). Ideally, Post #2 should be the same length as the legs and/or outspread wings of the insect. This is especially pertinent when working with materials that you wouldn't want to cut to length after tying them in (like deer hair tips). If you want a thorax, dub it or wind it on now.
With me so far? Okay then, here it comes. To make the hackle, pull Post #1 forward through Post #2. Post #2 will (or should) diffuse out to both sides, creating a flat hackle (Figure 3). Using a fibrous material for Post #1 may require that you twist the post BEFORE pulling it forward. The twisting makes Post #1 relatively solid, thus preventing materials in Post #2 from slipping through it. In addition, using softer materials for Post #2 may require that you render some assistance to get a good diffusion. Here, you can tie Post #1 down just behind the hook eye, being sure not to trap any material from Post #2. I typically clip post #1 so the end extends over the hook eye.

Diffusion Hackling can be used for a huge array of flies in a huge array of sizes and styles. Because of its versatility, I consider Diffusion Hackling to be a "must-have" technique. Give it a try the next time you're at the tying table.