This is the tricky part of the trick! Unfortunately this one might have to be lumped in to the irritating, but true category of, “You just have to keep practicing.” I’m not 100% hitting it either, but one thing that seems to help me is to push slightly down and away with my TH index as the yoyo is swinging up around my TH. This separates the strings just a bit. Still have to be accurate, and time it just right, though.
Once you start to hit it, go right into practicing Shockwave as a repeater. That way you can rep it over and over in a short period of time. GL!