Oftentimes you are alerted to the proper line of play by the opponents' apparent discarding problems.
I found myself in a rather inelegant 2 notrump contract yesterday at matchpoints, vul. vs. not. (Board 21).
I opened 1 spade, LHO overcalled 2 diamonds, partner made a negative double and I chose 2NT as one of my unappealing choices of bids. All passed and LHO led the king of diamonds.
I won the second diamond as RHO discarded an encouraging low heart. I didn't like my chances as I could only count 6 tricks (1S, 2H, 1D, 2C). I decided to lead a club out of my hand immediately, figuring if LHO had the ace he might be anxious to take it to cash his diamonds. I was right--he hastily grabbed his ace. On the run of the diamonds, I discarded 2 spades from my hand and a heart from the board, while RHO, oddly, discarded 2 more hearts and a club.
LHO now shifted to a heart and it was time to assemble the clues. What was going on here?
At this point there were only 3 hearts outstanding. Was it remotely possible that LHO would underlead Qxx? No. I therefore won the ace of hearts and cashed the king as RHO followed with her remaining queen doubleton. Now I had my 8 tricks: 1S, 3H, 1D, 3C.
There was only one rational reason why RHO discarded down to Qx of hearts--she had to be holding both the king and queen of spades and was squeezed. And so it was.
She started with KQxx/Qxxxx/x/xxx and needed to find 3 pitches. If she had kept her queen of hearts protected, she would have been squeezed in the majors when I played all of the clubs. She would have had to unguard either the hearts or the spades, and my simply playing my winners would reveal which of my major suits would run.
Of course, LHO can prevent all of this by ducking the first club and later winning my king with his ace. Then I won't have the necessary number of winners to cash to squeeze RHO.
Plus 120 was worth 80% of the matchpoints.
See you at the table!