Suppose you elect to declare. A heart lead would have left you with no play but West's choice of a diamond is reasonable. You insert dummy's jack and ruff East's ace. Cross to the table with a high heart and run the nine of clubs to West's ace. Win the heart return with the table's remaining honor and play the king of diamonds. East ruffs low, you overruff, discard dummy's heart loser on the king of clubs and start a crossruff. The defenders can score only two trump tricks and you make your contract.