When you are knocked against, it is vitally important that you are able to read and alter your hand for the purpose of laying off to your best advantage. Underknocks do not occur as frequently as gin so you need to be aware of how to handle yourself when you do have an underknock situation. In most cases underknocks are played for deliberately by players who are primarily defensive, and those who play against known knockers. In the latter case, the knockers are deliberately playing for underknocks in the hope of disrupting their opponent’s normal patterns of play. It is all very confusing at times, but it can be the most beneficial part of your game if you know when and how to do it.
The player using this strategy will accomplish his purpose by not knocking his own hand when possible, but by keeping his count to an absolute minimum to always be in a position to underknock his opponent. He also may be deliberately discarding cards which, if used by his opponent for melds, will always leave him with lay-offs. Then, if his opponent should knock, the cards being held for lay-offs will in most cases cause him to underknock his opponent.
The inadvertent underknocks generally come through lay-offs which are not definitely known to the underknocker, but are merely cards being held by him because they were considered unsafe, or for their offensive possibilities. Gin-offs, when you can lay off all of your cards to go gin after your opponent has knocked, usually occur when a player has been holding nine melded cards and his tenth card is a card that he knows his opponent needs.
Both underknocks and gin-offs will also occur to a great extent in cases where a player knows his opponent is on a schneid and must make every effort to win a small number of points for the purpose of getting off the schneid. It can also happen when a game is in jeopardy and must be protected from a gin by knocking at the first available opportunity.
When the score situation is such that it is known that an opponent will try to knock, it is advisable to always play your hand for the best underknock possibilities. In the reverse situation, if you are the player in the must-knock position, you must be aware of the fat that your opponent knows this and he will be playing for the underknock. Take every opportunity to defeat his purpose. When knocking, you should consider discarding, if possible, the fourth card off a known run to stop your opponent from laying-off. Knock in a way that affords the least possible lay-off, even at the expense of waiting a play or two. There will be many opportunities for using this strategy to counteract the strategy of an opponent playing for an underknock.
You should only try to underknock someone purposely multiple times if they are a known underknocker because it throws them off their game, or if you are way behind in the score. Otherwise, they will figure out your strategy soon, and you simply won’t be able to underknock without being “caught”.