The double figure eight knot is very fast to tie, and in my experience, a very secure knot. This knot is a good one for quickly tying of the rope to a piece of protection, or tying in climbers in sequence on glacier travel. These are only a couple of uses. You will no doubt find many other uses for the "Double Figure Eight Knot". Below is a four stop sequence showing how to tie this knot.
Double back the rope and create a loose loop as shown.
Next, turn the doubled rope to the left, and create a circle of rope as shown above.
Take the loose loop of rope that extends to the right in figure two, and wind it over the top of the double line.
Lastly, take the larger loop "A" in figure three, and wind it under the small doubled coil shown in figure three "B", and pull it through from beneath. Finally, pull the ends tight. You should now have a configuration that looks similar to the above.