Rules & Etiquette
If you give your partner any information about your hand - other than by what you have bid or played - this is unauthorised information.
What counts as 'unauthorised information'?
It could be any deviation from the normal pace of play - playing (especially a low card) or bidding (especially passing) too quickly may indicate that you don't have any strength in your hand - or, if you stop to think for too long before playing or bidding, this tells your partner that you have something to think about.
Equally, any 'attitude' in playing a card or making a bid could give information - a big sigh or a glare tells one story, dropping a card carelessly onto the table or paying little attention when the opponents are bidding/playing tells another.
You should always try to bid and play at a regular and steady pace, and to remain impassive - no matter how much you might want to dance with joy or scream with rage!
What's left? What information is 'authorised'?
Any bids that you make, any cards that you play, any agreed systems that you have. Most partnerships have agreed systems for count, attitude and discards - for example, discarding a low card asks your partner to lead that suit, playing high then low indicates an even number of cards. These are all perfectly legitimate, as long as you tell your opponents if they ask.
What happens if I do give unauthorised information?
There is nothing illegal about giving unauthorised information. Sometimes you can't help it - if you've got a really difficult decision to make, you might need to take some time to think about it. What is absolutely against the rules is using unauthorised information. If your partner has given anything away about his hand, you must not use this to your advantage.
For example, if your partner hesitates for a long time before playing a low card, you might assume that he has an honour in that suit as well. If you then led this suit back to your partner, your opponents would be well within their rights to call the director and claim that you had used unauthorised information. On another hand, your partner sighs or huffs and puffs while bidding a no trump, you might conclude that it is a bid of last resort and he doesn't have a good no trump holding. If, with a balanced hand, you correct this to a suit rather than leaving or supporting no trumps, your opponents could again claim that you have used information that you had no right to.
The director has a range of options available to him if he believes that this has happened, including changing the contract, changing the result and penalising the offending pair. It doesn't matter if the information was given deliberately or unintentionally - if the director believes it was used to gain advantage, he should take action.
A hesitation won't always give unauthorised information. Sometimes, a player may stop to think before bidding or playing, if he has two or more options available to him. It may not be clear what they are! For example, if you open 1NT and your partner thinks for a long time before bidding 2C (Stayman), you know that he has had to make a difficult decision, but you don't know what the other choices were - he could have been wondering whether he should pass, or whether he should jump straight to 3NT, or whether to play a transfer … you don't know any more detail about his hand than if he had made the bid immediately, so no unauthorised information has been given.
What if I had already decided to make that bid/play that card?
Sometimes you may read into your partner's attitude that he wants you to make a particular bid or play - and that may be the only sensible bid or play you can make! If your partner pauses for a long time before passing the opponents' 4S, he may have been thinking about doubling. Should you double? If you hold AK twice and another Ace, yes! Regardless of your partner's holding, you will double the contract - so you aren't using the fact that your partner may have hidden points.
What about holding AK twice, but no other Ace? Here, you're on very dubious ground. If the opponents have shown distributional hands, they are likely to have a singleton in one or both of your strong suits, so you are unlikely to make those 4 tricks. Unless you've got a lot of trumps as well, most directors would conclude that your hand on its own was not strong enough to double, and you had used the fact that your partner had shown hidden strength in deciding to double.
If your bid or play is not clear-cut, and you were thinking about maybe bidding/playing a card that your partner's unauthorised information indicates he would like you to bid/play, don't! Even if you were genuinely considering it as a sensible option, you may have to justify to the director why you did so - and if it's a borderline case, he may well rule against you. It's better to lose a few points on the score than to be branded a cheat, even if that wasn't your intention!
What about unauthorised information from the opponents?
Do so entirely at your own risk…
You can use your opponents' hesitations or attitudes to help decide on your bidding or line of play, but you will have no redress if you misread them! Some players stop to think even when holding just one card in the suit led, so it would be unwise to rely on them following suit the next time round.
However, you should not play at an uneven tempo deliberately, with the intention of misleading your opponents. This is definitely not in the spirit of the game!