The only type of falsecarding that is banned is purposeful falsecarding that a partnership has agreed to use in certain situations. For example, against notrump contracts, a player often leads the fourth-highest card of her longest suit. If she decides to make her opening lead from a suit such as A9753, and she and her partner have agreed to use fourth-highest leads, she would normally lead the 5. What she cannot do is tell her opponents that her side has agreed to fourth-highest leads, but then continually lead, say, her third-highest card. Obviously her partner is going to know what's going on after a while, whereas various declarers will always think her lead is fourth-best. This type of falsecarding is unethical.
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