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From Laws of Duplicate Bridge:
A player may not attempt to mislead an opponent by means of a remark or a gesture, by the haste or hesitancy of a call or play (as in hesitating before playing a singleton), the manner in which a call or play is made or by any purposeful deviation from correct procedure.
A player may appropriately attempt to deceive an opponent through a call or play (so long as the deception is not protected by concealed partnership understanding or experience).
Remedies for Inappropriate Deception:
When a violation of the Proprieties described in this Law results in damage to an innocent opponent, if the director determines that an innocent player has drawn a false inference from a remark, manner, tempo or the like of an opponent who has no demonstrable bridge reason for the action, and who could have known, at the time of the action, that the action could work to his benefit, the director shall award an adjusted score (see Law 12c).
Note: “Innocent” in the above context is a very important word. For example: Dummy has A K J 10 of a suit. Declarer leads the suit and LHO pauses for a long time. The one thing every bridge player knows is that LHO is NOT pondering whether or not to play the queen. A declarer who takes the finesse and claims he was misled by the hesitation is not considered “innocent” under this Law.