I sat on my balls a bit before seeing this thread, which is why I thought of it.

Definitely happens of you aren't careful sitting down. Feels a bit like squeezing a grape too hard

Its happened to me exactly once on a hard school chair and I still remember it, so uh yeah lol

Not inside the vagina, but the…bubble, for lack of a better term, can meander to the opening and sit there until you surreptitiously take a long stride. Might be what they mean.