Originally posted by Eladar
The pawn gets promoted before the opponent's next turn. This means that it should show the pawn turning into a knight and a knight getting captured.
At least that's how I see it.
I've just checked one of my games where an underpromotion happened. Upon underpromotion RHP does this (using underpromotion to bishop as an example):
If both bishops have already been captured one of the bishops in the captured piece sections disappears and a pawn appears, as though it were taken, upon recapture the bishop is retured to the captured piece section and the pawn still remains.