Streamer heads FP:
I think that the best way to get a handle on streamer heads and how they effect the fly's behaviour is by testing them on the water, and I hope that even after reading this you still will experiment and actually see how things work. I find it helps to split head styles into 4 main groups; slow, jerk, fast and jigging. Each head type will produce a different action on both articulated and unarticulated flies. So let's look at each of them individually.
"Slow" heads are the big hair, wool or dubbing heads that you see on a lot of modern streamers like the sex dungeon or silk kitty. This style of head is basically a waterbrake on the front hook which makes it slow down quicker than the rear between strips, causing the fly to jackknife as the rear hook continues to move forward. This can be a fantastic trigger to predatory fish.
The other place you might want a slow head is on single hooked flies, the muddler minnow is a classic example. Obviously, a single hook can't jack knife like an articulated rig but the head can make the fly kick to the side, creates a shimmy and pushes water helping fish to locate the fly.
Wool, deer hair and dubbing will perform a bit differently because of the different bouyancies but the general principle is the same.
"Jerk " heads are tall, narrow heads which usually sandwich a wing or collar material between a pair of eyes. A great example is Andreas Anderson's Delivery Man. This style still has some braking effect but the shape allows the fly to cut off to the side like a conventional angler's glide bait, great when you are targeting fish that respond well to being shown the full profile of the fly. With this head shape there's less difference between articulated and unarticulated versions than with slow heads.
"Fast" heads basically allow the whole fly to stop at the same time by being more or less the same diameter as the rest of the fly and may or may not include weight. Think cone head or Circus Peanut for these. Essentially when you are looking for a sinuous action this is the head for you. On articulated flies fast heads often include dumbbells or some other weight, but because of the size of the fly they generally aren't enough to make it jig, so what you get is a long gliding swim. The cone head peanut envy in the picture does this beautifully - there's enough drag in the size 1and 4 hook combination to stop the brass cone diving straight to the bottom.
When unweighted, these heads are the best choice for the stillwater steamers incorporating soft collapsible materials like marabou in my opinion, but choose something stiffer if you want to use this head style for swinging in faster current.
"Jig" heads are heavy and ideal for dredging or bottom hopping, barr's meat whistle is a great example. Jigheads want tungsten or lead most of the time, and definitely on articulated flies. This style is probably the most able to be influenced by the fly behind it so keep them mostly slim or lightly hackled or you can take a brute force approach and just pile on weight if you want a bulkier tail. These flies are great for species that feed on crayfish like bass or perch, keep them heavy and keep them on the bottom.
Hopefully you can use this a a starting point when you sit down to tie streamers. Understand what the heads do will get your fly close to where you want it to be, but you'll probably still want to tweak things after you've seen them in the water.