This is all part of the grand evolutionary plan that has made sure our bodies are programmed to freak out on a systemic level when we encounter trauma. Some people may feel faint or light-headed, weak, nauseated, or even pass out. This is called a vasovagal response, says Trost, and it occurs when the vagus nerves, which control heart rate, breathing, and blood pressure, and which are very sensitive to emotional response, are overstimulated. The vasovagal response can be triggered by lots of things — the sight of blood, fear of bodily trauma, and, in this case perhaps, severe pain.
Not every single person will have the same reaction, though. It is difficult to pin down exactly which autonomic systems are triggered when you get kicked in the balls because the testicles are “complicated from a nerve standpoint in that they have multiple different pathways, multiple different sources for pain, and take up a relatively large space in the brain,” Trost says.