What’s the most profitable opening bid?

On this deal from a Duplicate at the ARBC, East-West’s preemptive bidding catapulted South to the dizzy heights of 5 ♥. This would have been a bridge too far had West led a spade or diamond (able to lead a second round of that suit when in with the club), but in practice he reasonably led the ace of clubs.

At trick two West led the queen of clubs ruffed by declarer, who seemingly had three losers: the ace of spades and, unless East held a miracle ♦ Kx, the king of diamonds. Declarer cashed the ace of hearts, then led the nine to the king, preserving the six, so that he could shortly cross to dummy’s seven.

Declarer was pretty confident that East held both the ace of spades and the king of diamonds, because West had preempted and had revealed seven high-card points in clubs. At trick five he led a low spade towards his king, East correctly ducking (or declarer would have two spade tricks).

Declarer led the preserved six of hearts to dummy’s seven, then led a low diamond to the queen, the finesse as expected winning. Rather than cash the ace of diamonds in the (vain) hope that East’s king would drop doubleton, he started rattling off his trumps.

When declarer led his last trump and threw a spade from dummy to leave ♠ Q and ♦ J7 [in hand: ♠ 6 and ♦ A4], East was in some difficulty. What could he discard from ♠ A10 and ♦ K9?

If East threw the nine of diamonds, declarer could cash the ace felling East’s king and score his 11th trick with dummy’s promoted jack. In practice East threw away the ten of spades, but now declarer exited with a spade to dummy’s queen and East’s ace, awaiting the lead from ♦ K9, which he could run to dummy’s jack, scoring the last trick with his ace. 11 tricks and game made.

5 ♣ would have been just down two (-300 if doubled, cheap against 4 ♥ making), so South had to bid (and make) 5 ♥ to score well.