According to the official Laws of Duplicate Bridge (2017 version),
Law 68 C reads:
A claim should be accompanied at once by a clear statement of the line of play or defense through which the claimer proposes to win the tricks claimed, including the order in which the cards will be played. The player making the claim or concession faces his hand.
LAW 68 D reads:
After any claim or concession, play is suspended.
- If the claim or concession is agreed, Law 69 applies.
- If it is doubted by any player (dummy included); either
(a) the Director may immediately be summoned and no action should be taken pending his arrival, Law 70 applies; or
(b) upon the request of the non-claiming or non-conceding side, play may continue subject to the following:
(i) all four players must concur; otherwise the Director is summoned, who then proceeds as in (a) above.
(ii) the prior claim or concession is void and not subject to adjudication. Laws 16 and 50 do not apply, and the score subsequently obtained shall stand.
Law 70 reads
A. General Objective
In ruling on a contested claim or concession, the Director adjudicates the result of the board as equitably as possible to both sides, but any doubtful point as to a claim shall be resolved against the claimer. The Director proceeds
B. Clariﬁcation Statement Repeated
- The Director requires claimer to repeat the clariﬁcation statement he made at the time of his claim.
- Next, the Director hears the opponents’ objections to the claim (but the Director’s considerations are not limited only to the opponents’ objections).
- The Director may require players to put their remaining cards face up on the table.
D. Director’s Considerations
- The Director shall not accept from claimer any successful line of play not embraced in the original clariﬁcation statement if there is an alternative normal21 line of play that would be less successful.
- The Director does not accept any part of a defender’s claim that depends on his partner selecting a particular play from among alternative normal21 plays.
E. Unstated Line of Play
- The Director shall not accept from claimer any unstated line of play the success of which depends upon ﬁnding one opponent rather than the other with a particular card, unless an opponent failed to follow to the suit of that card before the claim was made, or would subsequently fail to follow to that suit on any normal21 line of play.
- The Regulating Authority may specify an order (e.g. “from the top down”) in which the Director shall deem a suit played if this was not clariﬁed in the statement of claim (but always subject to any other requirement of this Law).
This means that the claimer should state, right after the claim is made, how s/he proposes to play the hand, in sufficient detail so that no significant questions are left. This statement is binding. If any player thinks the claim is not correct, that person may suggest playing out the hand, and if all 4 players agree, play resumes. If anyone objects to the claim but there is not agreement to play out, the director is called and decides the result based on the claim statement, any stated objections, the previous play, and the lie of the cards. The claimer is not allowed to take a winning finesse not mentioned in the claim statement unless it would become marked before it needs to be taken on any normal line of play.
In general, any doubtful points are resolved against the claimer. Making a claim that one is unsure of is poor play, but claimers do make mistakes. Making a claim when the result is clear is considered good play and proper courtesy, leaving more time for subsequent hands. This is especially true in online play, where time limits are strict, so not making an obvious claim is considered poor play or poor sportsmanship.
Making a dubious but complex claim in hopes that the opponents will accept it rather than figure out whether it is valid or not is considered very poor sportsmanship, hardly better than cheating. However, I have seen this done in club play.
Prior to the 2017 laws the Director had to be called for all challenged claims, playing one out was against the law.
If the claimer's statement indicates that the claimer has made (will make) an error, as assessed by the Director, the non-claiming side gets the benefit of that error.