Clyde E Love made an analysis, and in most cases decided that for a squeeze to be present (from declarer's point of view), we must have a condition called "BLUE". Of course there are other types of squeezes (see: Adventures in Cardplay by Ottlik and Kelsey) which might not fall under this, but most of them do.
- B : All cards in victim's hand are Busy.
- L : Loser count is correct (Rectifying the count, typical one loser for most squeezes)
- U : At least one threat is in the Upper hand (lies in the hand that plays after the victim)
- E : The hand threat in the hand opposite the squeeze card has an Entry with it.
Defense can foil declarer by trying to disrupt any combination of the above.
Usually the defense depends on the hand, but to quote a few examples.
1) Defending by attacking "B".
Here defenders can play a threat suit repeatedly in order to nullify the threat.
(Suit order is spades, hearts, diamonds, club from top to bottom).
South is declarer in 5 spades.
West leads the Club K which east overtakes and plays back a club. Which west wins with the J.
Now the only defense is for west to play a third club, making declarer play the T, which East can ruff. This removed the threat card: CT.
Suppose West returned a heart instead of a club.
Then declarer could win the A, draw trumps, cash the diamonds and some trumps and come to this ending:
Now when declarer plays the last trump from hand, West is squeezed. If he throws a club, the CT becomes good, otherwise the heart 9 becomes good.
If West returns a club a trick three, he can hold onto his hearts and throw clubs, as there are no threats there.
2) Defending by attacking "L".
Here the defenders can hold up their trick to prevent declarer from rectifying the count.
A common example which is cited for this is the following:
South is playing 6NT.
Say West leads a spade and East should hold up. Declarer wins and play a spade. East should hold up again. Now if declarer plays another spade, East can cash two tricks. If declarer abandons spades, no matter what he does he cannot avoid losing two tricks.
If east takes either the first or second spade, declarer's count is rectified and West gets squeezed in the minors.
3) Defending against "U".
When both defenders can protect against a suit, they could potentially discard so that "U" is destroyed.
The below example is not exactly breaking "U", but shows how correct discarding can be important in making sure the right stopper lie over declarers threats, ensuring there is no victim!
Playing Matchpoints, South the declarer chooses to bid 7NT (grand slam in NT), instead of the cold grand slam of 7S.
West leads the Diamond 8. Declarer wins the K in dummy and plays another D to hand, East playing the J, showing the Q, but denying the T.
Now South cashes the club AK and starts playing spades. On the second and third round of spades, East throws the HQ and the H9, showing that he has the JT.
West knows that if he throws a heart on the spade, then East will come under pressure, as he will then have to protect both diamonds and hearts.
So West holds onto the hearts (throwing 2 diamonds on the 4th and 5th spade, and the club Q on the last spade).
East throws away all those hearts, and holds onto the DQ and the CT.
So West protects hearts, and East protects Diamonds and clubs.
(Note: there might be a better play for declarer, but this hand is there only as an example of how the right discarding can prevent squeezes on the defense).
4) Defending againt "E".
This is considered the most difficult squeeze defense. Trying to attack declarer's timing, sometimes by knocking out the entries required.
Here is an example:
You are East. Declarer overbids to a small slam in hearts, after partner overcalled 1S.
Partner leads the CQ.
You play too quickly without thinking to trick one and win the A of clubs (if you duck, you might be effecting a defense against "L"). But all is not lost.
You can still beat the contract, if you return a spade! right into dummy's AQ.
If you return anything else, partner will be squeezed in spades and diamonds.
Declarer can reach this end position:
Now when declarers plays the H9 (and will throw a club from dummy), west is squeezed. If he throws a spade, dummy becomes good. If not, the DT becomes good.
Playing a spade knocks out the entry to dummy and breaks up the squeeze, letting partner throw spades, and hold onto the diamond.