Ok this is what happens to me. When I get very nervous I have to go to the bathroom sometimes to get away from people. Recently I have been doing this a lot. I take a few deep breaths and I hold my breath and the blood rushes to my brain and this is when it happens. As the blood is rushing to my head I crouch down so I don't collapse. My hearing goes funny, like it's all broken up. Also I feel my eyes rolling about and I can't see anything. I don't know what's happening for a minute, then I regain consciousness and it's like waking from a long intense dream, though I can't remember it and I know it's only lasted a minute or so. Today I woke up and I was banging my head against the wall and my legs were shaking. It seems I can induce this myself by doing the thing with my breathing. You may wonder why I would want to do this, but I always feel better after it has happened. It's like all this nervous tension builds up inside me, and this is a way of releasing it.