South is in 3S, after East has made an unusual 2NT bid, indicating both minors
West leads the D10 to East's DA. East returns the DQ, won by Declarer's DK.
Declarer can see 4 tricks, and only 4 tricks, outside of trumps. It also seems likely that trumps will split badly and a trump will be lost. If so, it seems that Declarer could be held to 8 tricks. (In fact, as Declarer, I was held to 8 tricks.)
But, an extra trick is possible with a heart ruff. Declarer should plan on East having only 3 cards in the majors. He should take a slight risk, and lead to the HA, at trick 3. (Actually it is not much of a risk since, with a heart void, East might well have 3 spades, and hence trumps will subsequently drop.)
The HA wins, and a heart is led back towards the HK. (If East could ruff this, he is ruffing a loser, so he would discard, and the HK would win). But here East follows.
Now Declarer leads a third heart, won by West. West probably leads his last heart, hoping East can over-ruff Dummy. Declarer though can discard a diamond from Dummy on this, intending to ruff a diamond safely later. Declarer will lose just 1 more trick, a trump to West.
Actually, on this hand, with a good guess of the opposition hands, one can make 10 tricks. After winning the DK, Declarer wins CA, club ruff, HK, HA, club ruff, heart won by West. Declarer has won 6 tricks and the defence 2 tricks. West will probably return a heart, hoping East can over-ruff dummy, as above Declarer will discard a diamond. If West retains the lead, he has to lead a trump and Declarer makes SJ, diamond ruff, SA, SK. If East ruffs his partner's winning heart, he has to return a minor. Declarer will ruff a club with the SK, either immediately, or after ruffing a diamond in Dummy, and catch West's SQ, by leading trumps from hand.