The Weaver’s Knot
There are many ways in which to join yarn ends, one of the best being the weaver’s knot. The ends can be woven in, or snipped very closely, and rolled together to from a nearly invisible, strong join. This is also a good way to tie the drive band on a spinning wheel.
Make your first loop, with the end under and behind the thread. Hold it between your index and thumb.
Put your new thread under and up through the center of the loop made.
Put that thread behind the end piece of the looped thread.
Drop the thread down into the center of your index and thumb, making sure the thread is in front of this thread.
Drop it down in the loop top of thread, and under again.
Grasp the tail end and the ball thread with your left hand. And the tail and the other thread with your right and pull them together.
How to undo any weaver’s knot and know if you’ve tied it correctly
The key to knowing you’ve tied the weaver’s knot correctly is to be able to release or undo it. To undo it, you want to straighten out the thread that makes the “U”in the completed knot. No matter which way you tie it, there is one thread in a U-shape and the other thread winding itself around the first. Pull on both ends of that “U” thread-in opposite directions-to unbend it and straighten it out. The squiggly portion can be slipped right off, and even the squiggles relax so you have two fresh threads when you’re through.
Information from http://groups.msn.com/NeedleTattingTwo/weaversknot.msnw
which also shows additional step by step diagrams.