I had a lot of trouble learning the GB 20 knock out ... I w ill not go into how long ago that was or how old it makes me feel. Try setting it up with a reverse shuto to the lung points on the oposite arm. If you do not know what that means, feel free to contact me at email@example.com.
Once the set up is done, try hitting the point(LIGHTLY!!!) with a ridge hand rolling up. If you do not follow this, again, contact me.