Understand the Threats

This position arose in my fourth round game in this weekend's Eastern Washington Open. I had Black and won, then won my fifth round game as well, finishing with 4.5/5 and a tie for first in the event. This was the second time I won a weekend Swiss (see "Winning an Open").

White to move

After 20...Rf6

White played 21.Bb2? and resigned on move 25 when checkmate was inevitable.