Discards are often used as salesmen throughout the game of gin rummy. A salesman is a card that is discarded when you want you opponent to give you another one of the same denomination.
For instance, if you are holding the Q ♦, and the J ♦, the throw of a K ♠ (in this case, the salesman) will tell your opponent that you are not saving kings and it will frequently induce him to throw the K ♦ if he has it. This works because there is a natural tendency for a player to match a previous discard rather than throw a random card.
There are many ramifications to discarding a salesman. If your opponent picks up the salesman, it becomes highly probably that the card you want has become part of a spread, and you will never get it. Let’s look at the above cards. If your opponent took the K ♠, you then wouldn’t know if it was for the kings or a spade sequence. If it is for the kings, then you holding onto the Q ♦ and J ♦ would become worthless. If you know through play though that he has the K ♠, Q ♠, and J ♠ then you know you will probably get the K ♦ when he picks it. You also know that the Q ♦ and J ♦ are safe cards for you to throw if you have to.
If you use a salesman card too frequently or in an obvious way, your opponent will catch on and will be very careful not to give you the card you want the most. Since throwing a salesman goes back to the very start of the hand, most players know how to differentiate between the salesman card and a normal discard. The expert player will then resort to more sophisticated methods during his plays which can only lead to trouble for you.
That being said, the best time to throw a salesman is usually on your first or second discard because your salesman is almost always a wild card which your opponent might use for either a sequence or a group. Early in the game is really the only time that you should figure you can get away with it. Any time after your first couple of plays, the salesman card could be very dangerous to throw because by that time your opponent already has several ways in which he can match up his hand.
If you can do it and get away with it, it means that you will almost certainly win the hand, but if it goes wrong you could be left with more points then you wanted to have. Before using the salesman card you need to size up your opponent. If he is an expert player then using a salesman card once could essentially change the outcome of the entire game, but if you are playing an average player they might not catch on until you already are an odds on favorite to win.