Fuente: Julio 2014 ACBL Bridge Bulletin 
Defenders may sometimes feel trapped in a dilemma: Should they “do something” and try to “find” partner’s length and strength, or should they be passive and stay out of harm’s way? To treat this common headache, an understanding of frozen suits is critical.
In both of these examples, whichever side first leads the suit loses a trick. There are many more similar combinations. The idea is to recognize potentially frozen suits and, if you’re declaring the hand, find a way to force the defenders to open these suits. Unless they need to cash winners before declarer obtains discards, defenders want to lay off that suit.
For the most part, declarers like defenders who actively grab winners, underlead honors and shift suits. To defend passively at the right time is a hallmark of a good defender. You find yourself in the East seat defending 4:
South opened 1NT (12-14), North used Stayman, and over South’s 2 response, raised to 4. Partner leads the 2 (fourth best). Plan the defense.
The dummy, for all its high-card points, is not threatening. It contains no ruffing value, which might call for a trump switch, nor a long suit to generate discards, necessitating a cash-out defense. What are your options?
A club shift, hoping to find partner with the queen is an unnecessary risk. True, partner is marked with 3-5 HCP (add the HCP you see to declarer’s 12-14 and subtract from 40). If declarer holds the Q, you have lost your king. What’s wrong with a trump shift?
Suppose declarer holds A-K-10-x and partner has Q-x. Left to his own devices, declarer is apt to take the percentage-but-losing finesse. Your spade shift, however, may induce him to change his plan and play for the drop. It is entirely possible that partner holds Q-x or Q-x-x. If declarer holds A-9-x, you recognize that the heart suit is frozen, making a heart lead something to be avoided.
So you win the A, return a diamond and let nature take its course. If declarer takes a later club finesse to your king, remain passive and exit with a club. The idea is to return suits that have already been played, not strike out into virgin territory.
Unless clairvoyant in trumps, declarer is booked to lose one trick in each suit. Note the icy nature of the heart suit – whichever side leads it loses a trick. If declarer somehow rejects the club finesse and plays the ace-jack (hold your cards back), win the king and return a third club to declarer’s queen, patiently waiting for your side’s heart trick. Good things come to those who wait.