The Hitching Post

Playing matchpoints, you are the Dealer, the opponents are vulnerable and you hold:
♠ 8 5 ♥ K 10 6 3 ♦ Q ♣ 8 6 5 4 3 2

You	North	East	South
Pass	1 ♦	2 NT	Pass
?

Partner's overcall was the Unusual 2NT convention, showing the lower two unbid suits (hearts and clubs). Typically partner will be at least 5-5 in the specified suits; at this vulnerability partner could have a very weak hand, intending the bid primarily for its preemptive value.

What is your plan for this hand? What call do you make now? Considering the possible ways the auction might develop from this point, what call will you make after the opponents continue in diamonds, or start bidding spades? Take a moment to decide before you read on...
♠ 8 5 ♥ K 10 6 3 ♦ Q ♣ 8 6 5 4 3 2

Ready now? Excellent, but I have one more piece of information for you.

Partner "hitched". 

Slightly but unmistakably, just after she placed the 2 NT bid on the table: a momentary gasp, the slightest jerk of her hand as if to pick up the bid, then she regained her composure.

It appears that there might be a problem with partner's bid. Would you like to change your plan?

ANSWER: Don't you dare choose a different call! You know something about partner's hand -- her 2NT call has some sort of problem -- but you are not allowed to use that information. The only authorized ("legal") information that you have, and the only information you can use, is that partner bid 2 NT. You must make your call on that basis alone. Much like Ulysses, you must lash yourself to the mast. Even though these Sirens are trying to lure you to safety, you must ignore them.

You must choose your call as if partner had bid in a normal manner. Furthermore, any subsequent calls you make later in the auction must ignore partner's extraneous actions. Even if partner makes a seemingly-strange call later, you can interpret that call based only on the authorized information that you have. (Notice, thus, there is no ethical way to escape after an erroneous call that was accompanied by a "tell".)

After play ends, the opponents have the right to summon the Director if they think your choices were influenced by partner's extraneous actions. The Director will assign an adjusted score if he finds that your choices resulted in an unfair advantage.

Here's what the rule book has to say. From the Laws of Duplicate Bridge, 2017, Law 16 "Authorized and Unauthorized Information", with some emphasis added:

B. Extraneous Information from Partner
1. Any extraneous information from partner that might suggest a call or play is unauthorized.This includes remarks, questions, replies to questions, unexpected alerts or failures to alert, unmistakable hesitation, unwonted speed, special emphasis, tone, gesture, movement or mannerism.
(a) A player may not choose a call or play that is demonstrably suggested over another by unauthorized information if the other call or play is a logical alternative.
(b) A logical alternative is an action that a significant proportion of the class of players in question, using the methods of the partnership, would seriously consider, and some might select.
2. When a player considers that an opponent has made such information available and that damage could well result he may announce, unless prohibited by the Regulating Authority (which may require that the Director be called), that he reserves the right to summon the Director later (the opponents should summon the Director immediately if they dispute the fact that unauthorized information might have been conveyed).
3. When a player has substantial reason to believe that an opponent who had a logical alternative has chosen an action suggested by such information, he should summon the Director when play ends. The Director shall assign an adjusted score (see Law 12C1) if he considers that an infraction of law has resulted in an advantage for the offender.

BONUS MINUTE: So what should you bid with ♠8 5 ♥ K 10 6 3 ♦ Q ♣ 8 6 5 4 3 2  after partner's Unusual 2NT bid, at favorable vulnerability?
4♥ is the experts' choice here. You have at least a nine-card fit in hearts, and at least an eleven-card fit in clubs. The opponents therefore have a double fit in spades and diamonds. You have no defensive values, and the vulnerability is in your favor. Make it difficult for the opponents to figure out their double fit, and be ready to bid 5♣ over their 4♠.

-- Ray 
Better Bridge in 5 Minutes. Guaranteed! (or the next one is free)