A hackle looks like a comb, but really is not a fiber processing tool at all. The fiber you use with a hackle should already be in a 'ready to spin' state, because once loaded on the hackle you will pull it off, no combing. So whatever state your fiber is in, will pull off in the same state, or if it is not well prepped, be hard to pull off.
The best use for a hackle is to blend similiar length fibers into a top for spinning. In my experiment I was using a merino top and a hand carded dyed angora fiber (the blue in the photo). So the fibers do not have to be top to be used, roving is fine.
The fibers are laid onto the hackle exactly like you would load a comb. Layer the different fibers however you want. The photo below shows both the fibers loaded on the hackle and being pulled through a diz. That process is also exactly like you pull fibers off of combs after combing. So if you are already familiar with combing, you will not have any problem at all using a hackle.
A photo below showing the pulled off top.
Several questions come to mind as I write this. Could I use that hackle as a comb base? Well technically yes, but it wouldn't be good for two reasons, one you can not rotate the hackle like you do combs, and two you have to have a comb any way for combing on it, which means why use it for combs when you have combs. I suppose you could load not well prepped fiber on the hackle, comb it with a small hand comb across the hackle and then pull it off. It would help those that only have small hand combs to comb a larger amount of fiber at one time.
Second question, is a hackle really worth the price? I think mostly not really for most spinners. However there is one nice aspect to it, you can control just how thick or thin you pull off your blend by using a diz with different size holes. It would enable the spinner to spin very fine yarn for lace weight from the blend.
Last question, why not just do the blending on large combs? The main reason not to is that most large combs have more teeth in descending on them, as in the five pitch combs. This is necessary for the processing of combing out all the shorter fibers when you are prepping the fiber. That is not really the goal with the blending process. In fact there is actually waste left on the hackle, surprising for having already prepped fiber. Trying to blend on five pitch combs with increase that waste to probably unacceptable levels.
But basically the hackle is a simple to use but very gratifying tool for the spinners toolbox. There are endless variations to play with when it comes to blending fibers before spinning them, and that is what makes this process so much fun.
On a personal note: did you notice the better photos in this post? I got my much needed improved digital camera, and went through the learning process of how to use it for this post. I bought a Sony Cybershot and I think I am going to really like it.