N/EW		♠	A K 5
		♥	7 3 2
		♦	A 5 4
		♣	K 9 8 3
				♠	7 6 3
				♥	A K 10 6 5
				♦	J 3 2
				♣	Q 5

West leads the ♥9. How should East defend?
This concerns IMP play (team match). So East will do his utmost to defeat the contract. Overtricks and extra down tricks are relatively unimportant.

Solutions

East counts that his partner has 3-4 HCP at best, meaning that it is virtually impossible that West has a long suit that's good enough to defeat the contract (after all, EW would have to set up that suit first and after that West would still have to have an entry).
Furthermore North has a relatively good hand (aces and kings), so if East were to defend passively (sometimes that's a good idea) South would probably have no problem in making his contract.
So East has to hope South does not have four hearts to the ♥QJ — a double guard — but only three — a single guard. West has another heart then and East can, in principle, make four heart tricks. Furthermore East must hope that declarer cannot make nine tricks without surrendering the lead once.
If all these conditions are met, East can defeat the contract, provided he keeps communication with his partner open.
Suppose that East wins the first trick and clears the heart suit (♥K, ♥A and another heart). South wins his heart trick and, if he has to surrender the lead once (which is what East is hoping for), will try to lose that trick to West — because by now West is out of hearts. An example:

N/EW		♠	A K 5
		♥	7 3 2
		♦	A 5 4
		♣	K 9 8 3
♠	10 9 8 2			♠	7 6 3
♥	9 4			♥	A K 10 6 5
♦	Q 9 8 6 2			♦	J 3 2
♣	J 4 2			♣	Q 5
		♠	Q J 4
		♥	Q J 8
		♦	K 10 7
		♣	A 10 7 6

South counts seven top tricks (♠AKQ, ♦AK and ♣AK) and is certain to score a heart trick. That sums up to eight. Clearly the ninth trick has to come from the club suit. If South has won the second or third heart trick, his problems are over: he crosses to dummy's ♣K and continues with a small club. If East would contribute a small club, South would insert the ♣10. West would be welcome to score the ♣J, but would not be able to return a heart. Here East plays the ♣Q to the second club trick. South wins and returns the suit: he is lucky, since West has the master club.
East should foresee this and play a small heart to the first trick. South thus makes the first heart trick and West still has his second heart. In other words: EW still have communication in the heart suit. It doesn't matter now which defender wins the club trick South has to surrender: no matter what, East makes four heart tricks now. Down one.*

Many East players would not pass the test here. The reason is that defending is more difficult than declaring.
To prove that, we'll compare this defence with a situation in which declarer plays 3NT. This is a side suit, in which declarer needs to make four tricks:

		♦	A K 5 3 2
		♦	8 6

*PS: Suppose that in the problem deal East defends as recommended by ducking the heart lead. South can now put EW to the test by playing a heart back to the second trick, hoping that West comes under pressure when East runs the heart suit.  In this case, however, West shouldn't have a problem: apart from discarding two diamonds he can also let go a spade. After all, South will not have four spades, in view of his leap to 3NT.
South can do even better: after winning the heart lead, he can cash the ♠AKQ and then exit with a heart. After cashing four heart tricks East then has to choose whether to return a club or a diamond. If he chooses wrong — a club — South makes his contract after all.