It is an ancient design, found carved into churches, temples and tombs around the world.
They are also easy and fun to draw, once you have the key.
Step 1: The Key
Step 2: A Simple Key
Step 3: The Common Key...
Step 4: ... made less common.
This is also unicursal, but with a twist. Try it to find out why (don't join a line to the point marked "X").
Step 5: Some keys to try.
Try starting with the keys in the images below, experimenting with the ordering to see what happens to the path.
Step 6: Like Minds.
Some of their ideas are a bit "new age" for my taste (the "Energy Keepers" need ti get a better grip on reality, I think), but there is still lots of useful stuff on their site: