The Tie Fast Tool helps you tie nail knots which are most commonly used for tying new leaders onto fly lines or tying your backing onto a new fly line. It can be used for a variety of applications, but those are the most common uses for this tool. It is a must have piece of gear.
This video will show you how to tie your fly line onto your backing, or tie a new leader on. There are lots of good "how to's" on this video.