Understanding the unusual

At favourable vulnerability, you pick up the following:

SOUTH (you)
♠ A7654
♥ KT9
♦ 543
♣ K5

Your left hand opponent opens 1H and partner overcalls 2NT. This is the Unusual 2NT, showing at least 5-5 in the minors and (usually) a weak hand. East jumps to 4H and you decide to pass, due to the lack of a clear alternative. When it goes back to your partner, however, she backs in with a bid of 4NT, passed back to you for a decision.

WEST  NORTH  EAST  SOUTH
1H    2NT    4H    Pass
Pass  4NT    Pass  ?

Partner can not  suddenly have developed a strong enough hand to ask for aces so the 4NT bid cannot be Blackwood, it is still a form of the Unusual Notrump, but with more extreme length (to back in at this level). Having already shown both minors, why did North not just bid 5C to force you to pick a minor? The answer is that 5C would suggest that the clubs were at least as good as the diamonds. The 4NT bid, refusing to emphasise the clubs, shows that diamonds are the longer minor.

You bid 5D, passed around to East who ventures back in with 5H. You double - after all, you have a spade winner, a likely trump trick, and short enough minors that the opponents will hopefully not too soon be trumping any winners your side has there. That concludes the auction, and partner leads the S3.

               EAST (dummy)
               ♠ QJ92
               ♥ QJ876
               ♦ J
               ♣ Q32
        SOUTH (you)
        ♠ A7654
        ♥ KT9
        ♦ 543
        ♣ K5

Given that partner has shown extreme shape in the minors, her lead of the S3 must be a short suit lead. It cannot be high-low from a doubleton (as the only lower spade, the 2, is in dummy) so it must be a singleton. You take the SA but before you return a spade, you check the opponents' system card which confirms that the 1H opening promised five hearts, so partner has none so there is no point trying to give her a spade ruff.

The lead of a singleton when holding no trumps is a poor lead. Partner must have had no other good choice of lead - no touching honours, for example; probably her minors are headed by the ace without king (poor suits to lead from). Backing your judgement, you bang down CK, and sure enough this holds, while partner plays a high club to encourage. You lead your other club, and ruff the return. Then you lead a diamond, and partner wins the DA then leads another club, ruffed by dummy's HQ. You have the K-T of hearts remaining. Should you overruff?

Remembering partner's Unusual 2NT, you know declarer can have no more clubs left. If you overruff with the HK, declarer will beat you with the HA, then draw your last trump. Instead, you discard. That leaves you with the K-T of hearts sitting over dummy's HJ, so you will make another trump trick.

Four off, doubled and vulnerable, gives you a top board, beating even the ambitious North-South pair who bid to 6D, which makes easily thanks to the favourable breaks in the minors.

The complete hand:

        NORTH
        ♠ 3
        ♥ -
        ♦ AT98762
        ♣ AJ987
WEST           EAST
♠ KT8          ♠ QJ92
♥ A5432        ♥ QJ876
♦ KQ           ♦ J
♣ T64          ♣ Q32
       SOUTH (you)
       ♠ A7654
       ♥ KT9
       ♦ 543
       ♣ K5