Your side is in a forcing pass auction whenever:
1) You have shown game-forcing values (i.e. after a GF raise, or a 2/1 GF) [or better, i.e. if someone has made a slam try]
2) After your side has issued and accepted a game invitation.
3) After your side has opened 2C (assuming 2C is your system's strong, artificial bid, even if it is not 100% GF)
4) If either partner introduces a new suit at the 4 level, which could have been shown at the 3 level (ex: 1H-(P)-2H-(3C)-4D sets up a forcing pass auction; 3D in the same situation is just a game try, and does not set up a forcing pass)
5) On the auction 1Y-(X)-XX, your side is forced to at least 2NT or suit agreement at the 3 level (a new suit introduced at the 3 level is forcing)
6) On the auction 1NT-(2Y)-X, where the double is played as penalty, your side is forced to at least 2NT or suit agreement (the opponents do not play undoubled)
7) When either of you has made a penalty double at the 3 level or higher, you are in a forcing pass auction at any level (this does not include lead directing doubles) - Note: this means you should never double the only contract you expect to beat
As a general corallary, if one of you could have cue bid, and instead leaps to game, you are NOT in a forcing pass auction. For example, if the auction goes 1S-(3H)-4S-(5H)-? opener must decide whether to double or bid on, or risk letting the contract be passed out. Responder had the opportunity to cue bid 4H, if s/he wished to set up a forcing pass auction.
As to what different actions mean in a forcing pass situation, I believe the following is the most common expert agreement, when both sides have a known fit:
Double shows the worst possible hand/holding for declaring. Depending on the situation, this can mean either that the doubler has a minimum hand for the preceding auction, or that the doubler lacks control in the enemy suit sufficient to bid on. The latter generally applies to decisions involving bidding to the 6 level or higher, and/or where strength has been narrowly defined.
A direct bid in the agreed trump suit shows extra length and/or confirms a control in the opponents' suit (at the slam level), but denies significant extra strength.
A direct bid in a new suit (below the slam level) is a mild slam try.
A pass, followed by a pull to the trump suit, is a strong slam try.
A pass, followed by a pull to a new suit is a grand slam try.
(The last wo sentences both assume that the passer's partner doubles, when referring to a "pull")
Any of the slam tries promise sufficient control in the opponent's suit to bid to the suggested level, unless partner has shown that control (i.e if my partner splintered in the suit, I don't need to have a control to suggest going on to the 6 level).
A hand without clear direction can simply pass and abide by partner's decision.
On auctions where their side has not established a fit (i.e. someone stuck their neck on the chopping block, solo), double simply shows a good defensive holding in their suit, bidding on shows shortness in their suit and/or unexpected shape on the side (perhaps a 5-5 hand, even with a doubleton in their suit). Passing denies either unusual length or extreme shortness in their suit, and agian you can use pass-and-pull, or pass and then take a move if partner bids, to show a good hand. Pass followed by sitting for partners action (or signing off, if appropriate) shows a non-descript minimum.
Similarly, on auctions where your side has not yet established a fit, such as redouble auctions, double shows length/strength in their suit, direct bids show extra shape without extra high card strength (including jumps, which show extra playing strength), and passing then pulling a double shows extra shape and strength.