Can anybody explain this rule to me?

"Promoting a pawn: You can move (rather than drop) a pawn onto an eighth-rank square, but only if your opponent's prison contains a queen, rook, bishop or knight. The pawn at once vacates the square, going into the prison. You then carry out the promotion by filling the vacated square with whichever you choose of the queen, rook, bishop or knight."

So I can promote a pawn only to a piece which is contained in the opponent's prison? And the promoting pawn goes to the opponent's prison, being exchanged for the piece to promote to?