1. A forcing pass forces partner to bid on or to double the opponents. Often, when a player has to make the decision between doubling a sacrifice bid made by an opponent and bidding on, he may pass and let his partner make the final decision.
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After East’s strong response, it would be unthinkable to stop short of game or allow the opponents to play undoubled. Both West’s pass over 3♠ and East’s pass over 4♠ are forcing.
2. Systems utilising the opening call of ‘Pass’ as a positive bid. Such systems are legal only in very few competitions and you are unlikely ever to encounter them.