The knot just provides extra pressure on the string. Basically the string is squeezed between the knot and the side of the loop. This creates more friction and more pressure on the string.
It’s important that the knot be at the apex of the loop. If you look the string should end up sitting beside the knot. When you slide it on your finger, be sure you pull the loop tight. If you do it slowly, you can feel the knot kind of pop over each strand of the yoyo string. Take a look at the picture below and you’ll see what I mean.
For me the knot may feel loose when I first put it on my finger, but after my first throw or 2 it’s nice and snug and stays that way throughout play. I’ve been trying this for the last week and have tried it on several different yoyos and different string types, and haven’t had any problems with the string sliding around on my finger.
