How is that you had convulsions, but did not pass out? When I try, I always lose sight first (seeing stars/blackout), then my thoughts fade away and any type of awareness stops, and then the convulsions only start after. I notice the convulsions only when I release the pressure and as I start to regain full consciousness. Regarding your question, my biggest discovery was that there wasn't enough pressure. When I lean in as hard as I can, without holding back, it works wonderfully. Important: If you're just practicing, don't do this when the rope is fixed to an anchor point! Hold the other end in your hand instead, so the pressure automatically releases when you let go of the rope. Otherwise, you can die.