When partner, East, doubles the opening pre-empt of 3 diamonds, how good is your hand? You don't know the value of the diamond king but the rest of the honors in clubs and hearts seem to be worth their weight in gold. Partner doubles unaware of whether you had zero points or more, he just knows that you passed and cannot have an opening hand. Therefore you must show him that you have working values. One way to do this is to jump to 4 hearts. A bid of 3 hearts leaves him with a big question mark because, as big as his hand is, he still needs some working cards in your hand to take more than seven or so tricks.
We played this hand in the unit final of the North American Open Pairs competition and watched an experienced player respond with a very conservative 3 hearts, but partner made sure they bid the cold slam anyway. Another competent pair failed to reach the slam at all. Neither declarer made the cold 13 tricks that are available with no danger and no particular effort. After you take the spade king and cash the diamond ace, thus unblocking the suit, you come to your hand with a trump and ruff your low diamond with dummy's ace. Since both opponents followed suit to the trump, you know trumps are no worse than 4-1 and now cash the heart king in dummy and lead another heart to your hand to draw all the trumps. Now one of the small spades on the board goes away on the fourth round of hearts and the other on the diamond king, which is valuable after all, and all your clubs are good. Seven made!
Doubling and responding to doubles seems to be the last area of bridge knowledge in which learners become adept, so if you wish to improve your game quickly, learn your way around doubles. When you have more than 10 points, you must jump the bidding in response to partner's takeout double. A non-jump bid tells partner that you have fewer than 10 points, so if he keeps bidding, he must be showing a very large hand.