In partnership play, the major requirement to play an expert game of partner’s gin rummy is to protect your partner’s winning score, if possible, even at the cost of your own hand.
For example, if your partner has just won eight and gin and you know that your opponent is holding nine melded cards and you believe that he needs the 10♠ to gin his hand, you might have a decision to make if the worse case scenario happens. You are holding nine melded cards and the 4♦. Going to the deck you pick up the 10♠. What should you do? You don’t think for yourself in this case, you think for your partner and the total score of the game. You simply throw him the 10♠. It might sound crazy as you know he will gin his hand but by letting him gin his hand you have protected your partner’s gin by losing only four points. There, your team will get on score with four points and the boxes that go with it. In effect, it is equivalent to your actually winning the hand.
If you had retained the 10♠ in your hand and discard the 4♦, you would have committed the “cardinal sin” of partnership play. You are then showing your partner that you are out to play for yourself, instead of the benefit of the team. Keep in mind that the basic idea of a partnership game is for you and your partner to win a particular hand and get some score, even if it is only one point. This is called winning. You don’t have to win solely; because some of the times, you win won’t be enough to win the hand or the game. If you are fortunate enough to win your hand and your opponent win’s his hand that is great. Primarily though, as a partnership game, it is more important for the partners as a team to achieve a score, no matter who wins and who loses.
On the other hand, if your partner has lost his hand, your prime purpose must be not just to win your hand, but to take every step possible to win at least as many points as your partner has lost, or more. Winning your hand, but not winning as many points as your partner as lost merely cuts your losses. It does not put you in a winning position. Therefore, just as your opponent is playing now to protect the count that his partner has won, you are playing to do everything possible to keep him over that count. If you succeed, your team will wind up getting that particular score and box.