You can throw one Spade on a Diamond but you don't have a third Diamond on which to throw away Dummy's second losing Spade. West does though...

The trick here is to throw West in with the ♥ A when she has no choice but to lead a diamond on which you throw the third Spade whilst you ruff in hand, or lead away from the ♠ K allowing you to play the ♠ Q which holds. Card-by-card it goes:

Play the ♦ Q, West Covers and you ruff. You need to be back in hand to play the ♦ J and the best way is to ruff the ♣ Q (yes I know it's a winner but the entry to your hand is more important). Now play the ♦ J and throw the ♠ 2 from Dummy. Then exit with a Heart to West's ♥ A. She can now only lead a Diamond (Ruff and Discard ♠ 6) or a Spade allowing you to win a trick with the ♠ Q.

This is called an Elimination Play because you eliminate suit(s) before throwing a Player in, leaving them with no choice but to give a trick away.