pawn sacrifice

I saw that Richard was even on material in a late game position, when every piece counts.  Then he checked his opponent with a pawn.  But the pawn wasn't covered, so his opponent simply captured with his king and apparently went up a pawn for free.  (?)  Why did Richard sacrifice his pawn?  I didn't see it, and his opponent didn't either, but I knew this opponent was about to suffer the consequence.  Richard moved his knight, forking king and rook.  In the time it has taken me to write this post, the game will be over.  Two games won, six to go.