While playing MP a lot is riding on overtricks - taking one more trick than the average has a nice effect on your scorecard. While people tend to concentrate on their declarer play, there is just as much to gain with a good defense such as the following:
Partner leads the ♥Q against a seemingly normal 4♠ contract:
Not too surprisingly declarer wins the lead with the Ace on the dummy, and calls for the ♠3 from dummy. What now?
Even though the ♠ Jack is almost surely in declarer's hand, it is crucial to play low trump now. In addition to the possibility of a miss guess when partner holds the singleton 10, it has an other huge advantage: if the opponent plays another ♠, we can draw trumps only holding on to our ♠6. This way declarer might lose a ruff, and even after collecting the remaining one - ruffing a heart in dummy - we can force him to ruff one more heart with his last trump developing our small trump for an extra trick.
The complete board is:
We can see, that declarer has 2 ♠ tricks, 2 ♥ tricks, 4 ♦ tricks and 2 ♣ tricks in hand. So he has to make a ♥ ruff and then draw trumps - otherwise we get a ♦ ruff. Our answer to that is winning the 2nd trump with the king, cashing the trump Ace - so dummy has no more ♠s - and now we can play our 4th ♥:
Declarer is now in trouble:he either ruffs and our ♠ 6 will be good, or he loses this ♥ trick.
No big deal right? Just try not to feel too much pity for the innocent opponents getting only 20%.