Sudden Weakness In Hands, Unable To Clench Fist When There Is Voilent Laugh, Looking At Photos Of Surgeries And Autopsies. Reason?

When I was a child, my brother would make me laugh while doing homework, and my hands would grow weak to the point where I couldn't grasp my pencil to write. It still happens from time to time. Someone else asked about this here and was given this response: "The sudden feeling of weakness after laughing is likely to be due to hyperventilation associated with violent laughing. The same is likely to happen after sneezing." However, I've noticed I get the same feeling sometimes when I see something really gory, or while looking at photos of surgeries or autopsies. I don't feel faint or dizzy, and my breathing and heart rate remain normal. I just get the weak feeling in my hands - my right hand usually feels weaker than my left and I am unable to even clench my fist. I'm curious as to the cause of this?