This would happen occasionally when I was Thai-boxing. In the gym, when guys were sparring and going pretty hard, someone would time a good shin, or a knee, right into the lower ribs of their opponent - who would go right down. They'd be out for a bit, too... just as if they'd been head-shocked. There was a E.R. trauma nurse working out with us at the time who apparently asked a doc about it and the doc told him it had to do with all the blood flow and pressure regulation that takes place in/around the liver. Knowing how much blood flows through that area each second, it sounds plausible. Like attacking the carotid sinus, you cause the body's blood pressure system to attempt to reset, which briefly crashes the blood pressure, resulting in brain reboot.