Scottish author Kenneth Grahame wrote, "The strongest human instinct is to impart information; the second strongest is to resist it."
Any time a bridge player takes positive action, he is imparting information to the opposition. And if he ends up defending, he will have made the declarer-play easier for his opponent.
Today's deal is a good example. If you were South, how would you plan the play in four spades after the given auction? West leads his fourth-highest heart, East taking two tricks in the suit before shifting to a diamond.
If the responder bids one of a suit over a takeout double, it is forcing for one round. If he has 10 points or more, he will have a hand better suited for offense than defense, because with a defensive hand he would start with redouble.
Although West has only two high-card points, he has great distribution and good offensive potential. A game or profitable sacrifice in hearts is possible. And note that five hearts doubled goes down only two.
With a club loser to come, you must play the trump suit without loss. Normally, you would play off the ace and king, hoping the queen drops -- "eight ever, nine nearly never." But East's double announced at least three cards in each of the unbid suits. So, cash dummy's spade ace (in case West has the singleton queen), then play a spade to your jack, taking the finesse that you know will succeed.
Although in this deal East's takeout double was automatic, if you expect to end up defending, resist the temptation to bid.