Gregory (Scotland Yard detective): "Is there any other point to which you would wish to draw my attention?"
Holmes: "To the curious incident of the dog in the night-time."
Gregory: "The dog did nothing in the night-time."
Holmes: "That was the curious incident."

Many , many inferences at the bridge table are predicated on inaction. The negative inference. "If West had 'A', he would have done 'B'. He did not do 'B', therefore he does not have 'A'".

Often the most revealing auction starts, "Pass, Pass,Pass". You now know neither opponent has opening values. 

In this example, you reach 4 ♥ having opened in the fourth chair after three passes. West leads out ♣ AKQ and you ruff the third round.

West would have opened the bidding if he held ♥ A in addition to ♣ AKQ. East has ♥ A and your only chance of avoiding a second trump loser is for him to have a doubleton. You cross to ♠ A and lead ♥ 2 towards ♥ K, East playing (say) ♥ 10. You return ♥ 5 and duck in dummy . If it's your lucky day, East's ♥ A will "beat air" and you can win his return , cash ♥ Q and drawing West's (say) ♥ J and claim. 

Note that if you knew West held ♥ A, you would lead to ♥ Q and duck on the way back.