In competing, a player must be positive that he does not want to defend the hand. A double of the final contract is the ultimate statement that one wants to defend, and competition is the complete opposite.
We have seen that with a misfit with partner, we want to defend, and with a Golden fit or a double fit, we want to declare. When we have length in a suit bid by an opponent, even if they are playing in a different suit, we want to defend and lead trump if that is not opponent’s suit we have length in. If one has four or more trump, one wants to avoiding ruffing and to force declarer to ruff and shorten his own holding. This is called the forcing defense or tapping the declarer.
The bidding: East, playing five-card majors and better minor, opens his longer minor and South overcalls One Heart. West has nothing to say and passes. He has a stopper, possibly two, in hearts and 6 to 9 points but refrains from bidding One No Trump because it is an impossible contract to play. Five cards of his entire hand amount to zero tricks if he declares a One Notrump contract. Against One No Trump, North, perfunctorily, will lead a heart and South will switch to his second suit, diamonds, after winning his Queen of Hearts. Now West gets zero Heart tricks unless he can end play South.
When West and North pass, East doubles in case his partner is trap passing. He is not, and South bids perforce his second suit. West breathes a sigh of relief that he does not have to bid his three-card spade suit. He does not have enough to leave a double of One Heart in. To double a one-level contract for penalty, one’s side needs a total of 22 points and at least three trump tricks.
East and West are happy to defend because they could be -200, a matchpoint disaster, in anything they declare.