Whip finishers are probably the least used and least understood, basic tool in the average fly tyer's arsenal. This is the tool that creates the nice knot at the head of the fly that keeps the fly together. If your flies fall apart, it's most likely due to the whip knot you didn't do or didn't do right when you finished the fly.
Some fly tyers use their fingers to do a whip finish knot. Although this often provides an acceptable knot, it sometimes provides an un-acceptable knot. Learning to use a whip finisher is one of the basic skills of fly tying. Be assured that this is one skill you must learn to do right to be as good a tyer as possible.
There are two main styles of whip finishers. One is the Matarelli and the other is the Thompson. (You might see a simple whip finisher; it functions the same as the Matarelli.) While I'm not certain of the origins of these tools, I think it's reasonable to assume they bear the names of their inventors. Both styles can be used on a variety of flies, but each style has specific benefits when used on certain types of flies.
The Matarelli Whip Finisher is one of the finest fly tying tools ever made. This handy tool is made out of brass and stainless steel and will allow the fly tyer to make very neat and uniform heads by exact placement and spacing of the tying thread. While on the surface, this tool appears very difficult to use, it really a very simple and easy to use tool once you get the hang of it.
Video Instruction - YouTube
The Thompson style whip finisher ties the same knot the other whip finishers do, it just does it a little differently. This is a little more difficult tool to learn to use, but just as important a tool as the other style of whip finisher.
Thompson style whip finishers are best suited for work in small confined places. If you're trying to tie a whip knot on a small dry fly and don't want to tie the hackle down, this is the right tool for the job. This is the best choice of whip finishers if you're using a hackle guard to hold the hackle out of the way.
This type of whip finisher isn't very suitable for flies with bead heads or bullet heads. It won't maneuver around jig heads or lead eyes. It isn't suitable for flies that must be whip finished back away from the hook eye. It is the best whip finisher to use on hair heads and small hackled flies, because it won't bind the hair or hackle down like a Matarelli whip finisher will.
Video Instructions - YouTube
The finger whip-finish is probably a little slower than using a whip-finishing tool, but it does save time unless you have your tool always at hand. The finger whip-finish provides a lot more range than tools. It is easy to keep a wide loop and easily tie off far back of the eye on a big fly, while avoiding obstacles like wings and hackles etc. in front of it.
Video Instructions - YouTube