The bidding was
N E S W
1D P P 1H
Perhaps South should protect with 1S or 2D, but he did not, and 1H by West was the final contract.
North led the DA and switched to the SQ, won in Dummy with the SA.
Declarer now ruffed a diamond in hand with the H4. (It is not normally a good play to make an unnecessary ruff in the hand with the long trumps, but Declarer was probably influenced by a likely bad trump break since he had been allowed to play in 1H. He wanted to make sure of a trick with his small trump.)
Declarer continued with SK and a spade ruff in Dummy, then led the C10, won by the CQ. North now knew that South had the DJ, so was able to lead a low diamond, which Declarer ruffed with the H9. Declarer was now sure of his contract, and led a low club which South won with the CJ.
Declarer won South's trump return with the HA and played another club to North's CA.
Declarer's hand was now fairly clear to South, and he decided to ruff his partner's CA and lead the S9. Declarer must ruff with the HJ and is over-ruffed by North's HQ. If North now leads the DK, South has to ruff with his last trump - HK, and North will take the last trick with the H10, holding Declarer to his contract.
Note that if Declarer had not made the unnecessary diamond ruff, he would make 8 tricks fairly easily.