The bidding: South has a fairly ugly 13 HCP’s, but his shape dictates an opening. If partner has a decent fit with one of the long suits, the deck is a 28-point, not 40-point, deck and then game can be made with considerably less than 25 HCP’s.
South is not strong enough to open One Heart and rebid Two Spades, so he lies about length not strength and opens One Spade with the intention of rebidding Two Hearts. This keeps the auction at a necessary low level.
North chooses Spades because his partner bid them first and Spades will usually be longer than Hearts.
It is unfortunate that declarer plays the contract in a five-two not a six-three fit, but that is just the way it goes. Bidding systems do not work all the time, however, one must bid in a way that will be successful most of the time.
The Play: West leads the Diamond Queen and not his stiff Heart. Because he has four trump, he wants declarer to shorten his long-trump holding. Leading a stiff in declarer’s second suit is a very risky proposition and West finds the forcing defense as a good excuse not to lead his Heart.
East wins the Diamond Ace and continues a Diamond. Declarer is forced to ruff with his long-trump hand. Thus, it is called a forcing defense. When East gets in with the Heart King, he will lead another Diamond, and declarer will be tapped again and will have fewer trump than West.
If declarer gets a Heart lead, he will do a lot better. Here is your assignment. Take two decks of cards and deal both decks as shown in the figure. With one deck, play the hand with a Heart lead and with the other, play the hand with a Diamond lead and continuations. You will see the benefit of a forcing defense.
Note: South could have opened One Heart and Rebid Two Hearts hiding the spade suit completely unless partner has four of them and replies One Spade. Four Hearts is cold because a six card suit is less susceptible to a forcing defense while Four Spades will go down with good defense. However, telling partner one’s hand is single-suited and not two-suited is probably not the best approach.