Chess Tactics: Deflection

Here is another 3 min game of mine on fics. A nice tactic allows me to win a piece, and later the game.

The tactic arose after following position :

Here white played his rook to

Why was the move wrong?

Tactcis involved : Deflection, Fork, Move without lossing tempo/ with gain of tempo, Undefended Piece

Whats the best move for black.....Solve it