The alpine butterfly is a useful knot as it enables a loop to be tied in the middle of the rope. While you could use a figure-of-eight loop, it has the ability to unroll, and is less strong in this situation.

 Alpine butterfly - the finished product
Pass the length of rope across your palm. Alpine butterfly - slide 1
Wrap the rope around your hand to make a complete turn. Alpine butterfly - slide 2
And once more. Alpine butterfly - slide 3
Reach under the outermost loop with your other hand and pick up the middle loop Alpine butterfly - slide 4
Pull the middle loop out and over the other two loops. Alpine butterfly - slide 5
And then feed it back through the other two loops. Alpine butterfly - slide 5
Pull the three parts of the knot (the two ends plus the loop) to tighten and set. Alpine butterfly - slide 7