The one thing you absolutely need to know if you have tied in the thread and started on your fly is also how to finish the fly. At some point the thread has to be cut and obviously the stuff you've been tying in has to stay on the hook. There are a few different methods to do that. Some do it with a few half-hitches and a drop of head cement. Some do it by doing the last few wraps with a little super-glue on the thread. But the method used by most is undoubtedly the whip-finish knot.

The whip-finish is a tie-off version of the normal thread mount method. Just like the first few wraps lock the tag end to the hook when you start to tie the fly, the last few wraps will lock the tag end when the fly is finished. The difference - at the tie-off, the bobbin end will represent the positioned tag end and wrapping has to be done differently.

There are two basic types of tools for this. One has several different models which can go under many names, such as 'Simple', 'Standard', 'Thompson', 'English' and 'Classic'. They can look a little bit different from eachother, but the technique for using them is pretty much the same. The other is mostly refered to as the 'Matarelli' type. The biggest difference between the types, aside of the design, is that the 'Standard' type has a fixed handle and has to be rotated by the tier TO wrap the thread, while the Matarelli model instead rotates freely inside the handle and will follow AS the thread is wrapped. It's a "speedy" for sure.

Then there is of course the finger whip-finish. It's probably not as fast as the other two, but instead it does save you the time it takes to find the tool deeply buried in stuff on the bench. It also offers a lot more in range than the tools do. With your fingers, you can keep a wide loop and easily tie off far back on a big fly, still avoiding obstacles like wings and hackles etc. in front of it. There is no reason to not learn the finger whip-finish, just because you have got and master the tool.