West plays in 4H, and North leads the CK
Declarer wins with the CA, leads to HK, finesses the HJ and cashes the HA to draw trumps He then leads the SJ.
Declarer's aim is to discard his losing diamond on the 4th spade. South, holding SAK63, should see the danger of a possible discard on the 4th round of spades, and plan to prevent it. In order to play the 4th round, Declarer has two possible entries to Dummy - the 3rd round of spades and a club ruff. (South can prevent the first by holding up a master spade until the 3rd round, and the second by playing clubs, when in with the first master spade, to force an early use of Dummy's remaining trump.)
South should win the SJ with the SK and lead a club, for North to win and continue with a third club which Dummy has to ruff.
Dummy now leads the SQ, not a small one. (Declarer has deliberately chosen the Q to try to encourage the Defence to take this trick.) It is difficult for South to know whether to take or not. North played the S5 on the first round, which could be from S1085 or S54. The play of the SQ suggests Declarer does not hold the S10.
If South takes the SA the contract will be made and if he ducks then the contract will be defeated. But that is not at all clear to South when he has to decide.