The underknock is the most commonly overlooked part of Gin Rummy, but it is as important as the knock itself. When your opponent knocks, the card value is counted. Once your opponent puts a value on his unmatched cards you can then show your hand to everyone in the game and start assessing value to your unmatched cards. If after you meld your cards and lay off any additional unmatched cards to give you a total that is less than what your opponent knocked at, then you have what is considered to be an underknock. You then win the hand instead of the person who actually knocked.
This can be a bit confusing at first, but once you get the hang of it, it is quite easy to calculate. Let’s say for example that your opponent knocked with 5 points. That means his unmatched cards add up to 5 points. You then turn over a hand consisting of 2 melds of three cards each and 4 unmatched cards. You look at the knockers melds and notice that 3 of your unmatched cards can be laid off to his melds. That simply means that your cards are either continuing a run of the same suit that your opponent melded, or if they have 3 of the same card and you have the other one you can lay that off as well. After laying off your unmatched cards you may have a card that is worth fewer points than his 5 points. If you only have a 3 left, then you have successfully underknocked your opponent and you win the hand. If you find that you and the person who knocked have tied in unmatched card counts then you still win the hand.
If you are able to lay off all of your unmatched cards on an opponent’s knock hand, it establishes what is called a gin off. Basically you have gin because you were able to reduce your hand to zero. This generally happens when you have 9 cards consisting of 3 melds and 1 unmatched card, but if you are lucky then it can happen with as many unmatched cards as you have. Usually though, you may have a feeling that your opponent needed the one card you had left so you choose not to knock in order to surprise your opponent with the gin off. This unmatched card is then laid off for the equivalent of gin. Both the underknock and the gin off are extremely important when it comes to points, because you get extra points for an underknock.
Underknocking is considered the skill part of the game. Many people when playing Gin Rummy will purposely leave one unmatched card until they can either call gin or underknock their opponents because of the surprise element to the game. This is an especially popular part of the game when the knock card is valued at 7 or above because it gives you more of a chance of underknocking your opponent.