I've used this term before in a different context: It's what happens when someone is about to do something that both scares and excites them at the same time. Like when a person suddenly finds themselves extremely attracted to someone and they want to make a good impression. That's when their brain seems to be both there and not there at the same time.

When observing someone in this sort of situation you quickly come to the conclusion that the brain has gone but then later--upon reflection--it may seem like it may have actually been present. The only way to know for sure is to find out how the events eventually concluded; opening the box as it were.

That's when you find out whether or not the person was a pussy.