To tweak out is to loose control of your body, fall to the ground, and start making strange motions and noises(Not really passing out, just loose control with conciseness). Normally happens when large amounts of marijuana are in the body, the person attempts to stand and get a huge head rush, thus falls over. This can be strange seeing, but don't worry, pot can't kill ya :)
Woah dude, head rush... He mumbled as he stumbled and crashed on the book shelf. We all laughed as he tweak out on the ground.