The bidding: South, with 14 points, opens one diamond. West overcalls one spade. This is both lead-directive and suggestive of a suit to compete with. North has the unbid suits and makes what is called a negative double. With 10 points and five hearts, North would bid two hearts, but when these conditions are not fully met, he makes a negative double.
This shows four clubs and four or more hearts and perhaps only six to nine points. If the negative double forces partner to the three level, it then promises ten points..
East stretches his bid of two spades, perhaps because of 4 card support and a doubleton heart. South with a minimum hand and four hearts would either double or pass, but his hand is good so he bids three hearts.
North can reevaluate his hand knowing they have a heart fit and raise to game. Shortness are not counted until the partnership has a fit and length points are only counted if the length is useful.
Had East passed and not bid two spades, South would have jumped to three hearts.
The contract: Four hearts by South
The opening lead: The ace of spades
It is always good to lead ace from ace king and not from ace empty.
The play: West leads the ace of spades and continues with the king which South ruffs. South draws trump by running the nine through. There are no trump losers.
If West switched to clubs, South would play him for the jack not the king, because with the king he would find another lead. South has no intention of touching clubs himself. He cashes the top two diamonds, pitching a club from dummy. He then plays the jack of diamonds. It is covered so he ruffs it and returns to his hand with a spade ruff. South pitches the rest of the club losers on the diamonds. Had West not covered the jack of diamonds, he would have pitched a club. This is our trusty ruffing finesse.
The result: Four hearts making six for +680.