I love this join! It’s a great join for yarns that are not too slippery. Once the magic knot is made, it’s usually simple enough to hide it within your work. However, there are times when it doesn’t work, so try it out first and see what you think. Most of the time when you find a knot already in a ball of yarn, it is NOT sturdy enough to keep, so you have to cut it out and join the yarn in another manner. This is one option for that.
You can read about how to do it, see process photos, and even watch a video here: http://jessieathome.com/stitchopedia-magic-knot/