Analysis:
Unless the diamonds are 4-0 we can take five tricks in the suit.
But now that the defence has removed our only side-suit entry to dummy we should be thinking about what cards will be played on the short hand's last trick in our long suit – the fourth round of diamonds. We need to win it in dummy, and that can only happen with dummy's ♦5 and the ♦3 from hand. So we must unblock by playing ♦9, ♦8 and ♦6 on dummy's three masters.
Note that we must work this out immediately. If we lead A♦ and casually play the three from hand, we are already doomed. We should always check when running a long suit, especially when it has low length cards.
