There are a number of techniques which get the trademark Spey hackle effect to happen properly. First and foremost is hackle selection. Some guys out in the PNW used to settle for Schlappen. I couldn't do it
Schlappen is very effective for certain applications due to it's liveliness in water, my favorite is Brian Lencho's technique of palmering it into the body of a bunny fly to provide color diversity and a radial fiber ribbing (for lack of better words).
But for Spey hackle, schkip the Schlappen. I prefer Blue eared pheasant, dyed or natural; then dyed Spey hackle (Spirit River, other vendors makred SPEY hackle) made from chemically burned and dyed (flank?) feathers from fowl; then dyed mallard - in that order.
Having tied quite a few Spey flies (but by no means an expert) I've noted the following tips/techniques:
Work with the natural curvature of the feather. The barbs curve like a hand, so the stem must wrap to position the barbs in the classic curved formation that identifies these flies.
Strip one side unless you are tying "working" flies for sink tips in turbid water and trying to save time. Certainly for summer flies or fall low water Speys, strip the side you are not going to hackle from the stem.
Leave a little triangle on the tip of the feather to anchor the hackle before wrapping. If you strip the wrong half of the feather, you can simply wrap it in an opposite vector. Don't worry about it if you do, cross-wraps are common with salmon and Spey flies. Just be careful how you tie it in at the end.
The stem is very important, because if it's brittle it won't lay on the body of the fly well and may even snap while hackling. One negative nuance of the chemically prepared dyed fowl is that the stem is often brittle or thick. It does however provide a broad range of colors and lengths to work with.
Palmering over seal fur dubbing makes for a beautiful fly and hides the roots of the hackle from view.
Pick the dubbing and hackles to form the right effect, and I often need to remove some of the barbs from the upper half of the fly to allow for the matched mallard flank wings to set right.
Given three types of flies:
Black Heron, Sol Duc Spey (see Bob Veverka's image on Expert*ise) and my summer shrimp Spey, I use three different hackles: Blue eared or black dyed fowl for large sizes, dyed mallard for small sizes, orange dyed fowl, and white dyed fowl - respectively. If you find the "perfect" schlappen hackles, they will make better choices than the dyed fowl Spey hackles. This has been a problem for me over the years.
Let's get together and twist up a few one of these days.