just start with a double square knot (or whatever the first step is called) and pull it tight before starting the bow. it won't move while you tie the bow, and it's so much easier than this. if you have particularly slippery substrate, start with a triple instead
A reef knot (or square knot) cannot be adjusted or tied under load. When you tie the second crossing, the first one always loosens a bit. The lapp knot in the video can be tensioned properly after tying, which is pretty slick.
