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.
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.