Oscar Wilde said, "One should always play fairly when one has the winning cards."
That is reasonable. But when one is in potential danger, falsecarding at the right moment may make the difference between success and failure.
In this deal, how should South play in three no-trump after West leads his fourth-highest heart and East puts up the queen?
South starts with six top tricks: four spades and two hearts. There are four more available in clubs. Is there any danger?
Yes, if East has the club ace and thinks to shift to a high diamond, with West having that ace hovering over declarer's king, South could lose one club and at least four diamonds.
However, declarer does have the advantage that defenders "always return partner's suit" in no-trump. Still, how can declarer push East firmly in that direction?
South must take the first trick with his heart king, not the ace. To win the first trick in no-trump with an ace is an advertisement that you are not worried about that suit, because if your only high card were the ace, you would have made a holdup play, not taken the ace until, probably, the third round of the suit. Then declarer plays a club. It would be almost psychic of East to find the diamond-10 shift.
Finally, note East is unlucky that he does not have ace-third of clubs. Then he could hold up that ace until he could get a signal from his partner. Here, West would immediately discard a heart to tell his partner that they cannot run the suit.