W/All		♠	A K Q 7 6
		♥	K J 10
		♦	Q 6 3
		♣	A K
♠	5
♥	A 7 3
♦	A K J 8 4
♣	Q 10 8 2

West leads the ♦A, East contributing the ♦7 and South the ♦5. EW use classical signals, so high-low is a 'come on'-signal.
How should West defend?

From dummy it's clear that EW can make five tricks at best: ♦AK, ♥AQ (if East has the ♥Q, the only important honour card West cannot see) and a diamond ruff.
West has to assume that South's ♦5 was not a singleton, for if it was, the contract cannot be defeated. By the way, South is bound to have at least one more diamond, since if the ♦5 was a singleton, East would have started with ♦10972. He wouldn't have encouraged with the ♦7 then, since he can see that South would ruff West's continuation of the ♦K, leaving dummy's ♦Q a master.

All right: East has three diamonds at most. In three cases he will play the ♦7 to the first trick:
1. He has started with the bare ♦7 or the ♦72 doubleton.
In that case West must continue the suit, giving East a ruff at the third trick. ♥A is the setting trick (if East has the ♥Q and South three or more hearts, EW will even collect two down tricks).
If, at that diamond layout, West were to play the ♥A and another heart (or at once a low heart), he would give away the contract if South has the ♥Q.
2. East has started with ♦1097.
In that case West must play the ♥A and another heart (or at once a low heart), hoping that East has the ♥Q.
If, at that diamond lay-out, West were to play the ♦K, he would set up dummy's ♦Q. If South has three small hearts, his third heart would disappear on that established ♦Q.

What is West to choose?

The solution is that West doesn't have to choose at all. He asks his partner by playing the ♥A (true, he has to hope that East can give a clear signal to this trick; if not — very unlikely — West will have to guess after all).
In case 1 East will play his lowest heart if he doesn't have the ♥Q. West now plays the ♦K and another diamond, East ruffing.
In case 2 East will play a high heart if he has the ♥Q. West plays another heart and East will make the ♥Q sooner or later (if declarer goes up with the ♥K and later plays a diamond from South, West will of course go up with the ♦K and play a heart to East's ♥Q).

The latter is the case below: on the ♥A East will convey a clear message by playing the ♥9: 'please, play another heart!'

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

For connoisseurs: what if East is short in diamonds and has the ♥Q?
- If he has started with the bare ♦7 and the ♥Q, the best he can do is to play his lowest heart on the ♥A. He ruffs the third diamond and even if South were to overruff, that wouldn't be a problem: East patiently waits for his ♥Q to win the setting trick (this defence will only fail if West has eight diamonds and South exactly three hearts).
- if he has the ♦72 and the ♥Q, East defends similarly, only losing if West has seven diamonds and South exactly three hearts.

And if East has started with ♦1097 and does not have the ♥Q (so he signals low on the ♥A), it's all futile: the contract cannot be defeated.