I go through this alot of the time just standing up, I get too excited and get up way too fast. Then I have to get right back down again and try again... It feels like it gets very close to a "lights out" situation, meaning soon as it happens I have to get the fuck down and "hug the ground" (bring my body to the lowest potential energy state in case of a fall). So, will have to agree with the instant lights out theory. No blood, no thinky thoughts.