When not to overruff

[rod]

When an opponent ruffs our winner, it seems natural to overruff when we can. However that's not always right. Consider this end position with hearts trumps.

♠ K
♥ --
♦ 86
♣ --

♠ 3
♥ --
♦ K3
♣ --

♠ --
♥ J10
♦ A
♣ --

♠ --
♥ Q7
♦ 2
♣ --

You lead your spade winner from the dummy and East ruffs. If you overruff you still have to lose a trump and a diamond. But if you discard the diamond loser, the rest are yours.

Seems simple but I have a hard time describing a principle to remember. Can you?