Assume you are playing Standard American (a natural system with 15-17 1NT openings and 5-card majors), IMPS, and nobody is vulnerable.
Some background. Usually when you pass, you are advertising weakness or the lack of desire to bid or compete higher. However, there are times when a pass says that you want to do something but you don't know what to do. How does partner know the difference?
A pass is forcing when it is clear that your side has a decent majority of strength, and you are in a situation that both partners know that, because you do have the majority of strength, that you are going to win the contract, or that you are going to double their final contract. If you pass and partner passes, that didn't happen; your pass tells your partner he must take some action.
You have to have an agreement with partner on when the partnership thinks it has the majority of strength.
The following situations may be included or excluded by partnership agreement:
(A) One partner has made a bid that forces the side to game.
(B) One partner has shown game-invitational strength and has not shown a fit. (A standard 1H P 2D shows 11 or more points so I would say that is enough to say the side has the majority of strength. 1H P 3H says the same thing but it's much more likely that some of the strength comes from shortness, so opener could easily have 11 HCP and six hearts and responder could easily have 8 HCP and a singleton spade, so most partnerships do not say that this confirms the majority of strength. Also, 1C (P) 1H (1S) 3H shows an invitational hand for opener (17ish) but since 3 points could come from a singleton, there's a decent chance that your side does not have the opponents outgunned in power if responder is dead minimum.
While a lot of the points from 1H P 2D could come from distribution, I believe that you are better off in the long run if you say that confirms a majority of strength. Some pairs will disagree. You should discuss with a regular partner.
© Our side has bid game voluntarily using a strength-showing bid. 1C (1H) 1S (2H) 4S shows about 20 points - maybe some come from distribution but you should still have them outgunned. 1S P 4S, typically a preempt or "weak freak" does not establish the majority of strength because the 4S bid is not strength-showing. 1C (1H) 1S (4H) 4S isn't necessarily strength showing since you had to bid 4S to take it away from the opponents; you could be quite distributional.
(D) One opponent is a passed hand and the other preempted and one of us has shown strength. For example:
Even though East has only passed, the opponents are pretty clearly not bidding 5C to make and it makes sense for East-West to buy the contract or double North-South. If East passes, he is expressing doubt (probably between doubling 5C and bidding 5S; so East is saying he might have a suitable hand for E-W to play in 5S if West so desires.)
There are probably other situations as well. For example, you can say that opening 2C specifies the ownership of the hand and creates a forcing pass situation if the opponents compete.
If you think your partner doesn't know about forcing passes or won't respect the forcing nature of your forcing pass, you might have to act even though it's a pure guess, even when your side obviously has the balance of power.
Also, and I can't stress this enough, each partnership must decide which of the above situations (and any other situations) defines the balance of power and creates a forcing pass situation if the opponents compete. I personally would play all of (A), (B), © and (D) as defining that our side has the balance of power. Many players including experts will disagree. But if you can determine that your side has at least 25 HCP and both partners know that, all partnerships would (or should) agree that passes should be forcing.
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Assume your side plays negative doubles so your partner's 1S call shows five spades.
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