While it is illegal to kick partner under the table, hum "Diamonds are a Girl's Best Friend," or scratch your right ear when you want partner to lead a spade, it is perfectly legal to have an understanding with partner ( which you must reveal to you friendly opponents, should they politely enquire) about what the play of certain cards implies. As we saw in tip one below, playing a high card in a suit, followed by a low card, indicates an even number, probably two, so that you are looking to trump the next card in that suit when/if it is played. You hope partner has noticed, so that she will play a third card in that suit and you can enjoy trumping in. It is therefore important that, having agreed what your signals and discards mean, that you watch what partner plays very carefully, as partner will get quite frustrated if she has been diligently signalling and you have not noticed. When partner leads a suit, she will want to know whether she should continue or switch to a different suit when she is next on lead. Sometimes this will be obvious from looking at dummy or from what declarer plays, but you can help.
Most people agree that playing a high card on partner's lead says you like her suit, and a low card that you dislike it. So, if partner leads A♥ , and you hold Q92♥ , you will play 9♥, encouraging her to continue with the King and another heart so that you can take your Q♥ before declarer can get rid of his third heart on a long suit. But wait, I hear you ask, doesn't the play of 9♥ followed by 2♥ show an even number of cards? Yes, technically it does, but it also says that you like hearts - you will either be trumping them, or winning your Queen on the third trick. So.....a high card in a suit is encouraging, but it could mean more than one thing. You may have two, and want a ruff, or you may want to establish a winner in the suit. Either way, you will play high first. Suppose you hold Q32♥? Life is not perfect! You just have to hope that partner recognises 3♥ as a high card! You certainly can't plonk down Q♥ as an encouraging card as you will be giving away a trick.
Whatever system of signals and discards you agree with partner, there will be times when you don't have the right cards to play, but this will not happen as often as you will have the chance to give partner some potentially useful information. Remember that negative information can be as useful as positive. If partner leads a suit and you have four small ones and don't want her to continue, play your lowest card, discouraging, and she may switch to the suit you really want her to play. It is true that you are not giving her the correct count (you should technically play a high card to show an even number - in this case 4, which might be useful in a NT contract)) but you don't want her to think you have only two and want a ruff - you want her to switch to a better suit asap., so play low. This is called an attitude signal (whether you like or hate a suit) as opposed to a count signal (showing how many you have). On the whole, most players use attitude signals on partner's lead, though some experts prefer to have count, as they can then work out the distribution of the hand, but that is something to worry about a little further down the line. Having looked at what cards you might play on partner's lead, we'll look next time at your own lead, and what information you might convey to partner by the cards you lead.