Source: http://www.vba.asn.au
Suppose you are a defender and have a trump holding of something like:
Qxx, or Kxx, or Axx
and you are sitting over declarer’s trumps. Partner leads a side suit in which both you and declarer are void, and declarer ruffs with a middling card, which you could overruff with your honour. The tip is:
[box type=»tick»]Don’t over-ruff with a sure trump trick[/box]
Discard instead. Why? Here’s an example to illustrate the concept:
Hearts are trumps, East leads a suit in which both you (West) and declarer (South) are void. South ruffs with 
J. If you overruff with the ace, that will be your only trick in trumps. But if you discard, your A10x is now worth two tricks.
That 10 made it fairly easy to visualise the scenario. But it can also occur when you have much lower spots. You are West here:
You decide to lead your 
7 and lo and behold, partner wins the K and A, as declarer follows. Now partner leads a third club, and declarer ruffs with the Q.
Your 7 is the setting trick, but you have to protect it. By discarding now, rather than overruffing, you keep your trump holding intact, whilst declarer’s has been weakened. Let’s see what happens next.
The full deal:
After declarer’s Q wins, he will cross over to dummy’s 
A and play a heart to the jack. This you pounce on with the king. It looks like partner has the 
A, so you play a diamond, partner wins, and plays a deadly fourth round of clubs in this position:
If declarer ruffs low, you overruff. If declarer ruffs with the A or 10, then your 7 has been promoted. Either way, your 7 will take a trick, not what you were expecting when you first started out on this hand.
Follow through the play if you instead overruff on the third round of clubs. Even if you play a diamond to partner’s ace, declarer can ruff the fourth round of clubs with the A, and draw all the remaining trumps with the jack and 10.