The pin of light you can see on a girl between the thighs and right at the vag. The light you hope to be able to see, because you know a number of things from the existence of that light:
1. She's at the very least in shape where it matters most
2. The area is properly ventilated
3. It's going to be tight
This term predates the misinformed "thigh gap" as it is preferred to have the thighs touch towards the top but have that pin of light exist. It has become lost knowledge that must be re-released to the masses.
You see that girl in the stretchy sweatpants walking away from us? Look at the top of her legs. You see that light at the end of the tunnel?
Yeah.
THAT'S what you want. You make sure the girl you're going for has that, and you'll always be in a good spot. Guaranteed.
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.