When the opponents interfere, they unfortunately take away bidding room, but they do open the door for such bids as passes, doubles and cuebids. These are not available when the opponents are silent. Pass when partner is getting another bid is the hardest bid to make of all those bids.
An important law of competition is the Law of Total Tricks. The part of the law, that one needs to know, is that one can compete to the three-level with a nine-card fit, and it either will make or be a successful sacrifice. With an eight-card fit, it is the two-level and a ten-card fit, it is the four-level, et cetera.
With a nine-card fit, the opponents are guaranteed an eight-card fit of their own. Furthermore, in competition, the three-level and the five-level are for the opponents. You push them there and leave them there.
The bidding: East passes, and South, with 12 HCP’s and five hearts opens One Heart. To make an overcall, not vulnerable, West needs eight HCP’s and a good suit. If the overcaller has less than opening, he is possibly making a lead-directive bid, and the suit must be good, KJ10xx at the least.
North has seven effective points, four for the ace and three for the singleton with four-card support. The Queen of spades and Jack of clubs may as well be nines for a contract in hearts.
East has nine points, eight and a doubleton heart. The Queen of hearts may just as well be the nine as well. The queen will fall under the opponent’s Ace and King and will not even serve to promote a third round trick in partner’s hand.
The bidding gets back to South at Two Spades. He does not have a bid other than pass because one does not tell the same story twice. He has already told North that he has an opening (bare) and five hearts.
If South bids in front of his partner, (which means he can pass and his partner still gets a bid), South is showing either extra length or extra values of which he has neither. South passes and North, knowing they have a nine-card fit, competes to the three-level. A perfect example of the Law at work.
The Play: When East leads the ace of spades, he will not want to continue spades because he sees the Queen in dummy, and he knows declarer has at most two spades and can make a discard on the Queen of spades. He must switch and because there is no future in clubs, he switches to the King of diamonds. South will duck and get the timing to lead up to the Queen of spades for a diamond pitch, thus making ten tricks.