|The Big Model|
Yeah, it's hard for me to describe this nicely, but I have to because Force doesn't have to be a bad thing.
Look, face it, you say "I play my character," but the fact is that your character can show up at certain places and at certain times because someone else said so. Or your character can have certain opportunities not because you made him or her go for it, but because someone else tossed it to you in a "now's the time" way.
When they do these things in a way which significantly affects how the story turns out (or even what it is), then that's Force. When that's OK by you, and even perhaps part of a kind of co-play for your character that you enjoy, then go for it.
Force can really suck, when it violates what you thought were established boundaries for who says what in your game - that's Railroading, pure and simple. Such applications typically undermine Story Now play in particular.
The Technique of control over characters' thematically-significant decisions by anyone who is not the character's player. When Force is applied in a manner which disrupts the Social Contract, the result is Railroading. Originally called "GM-oomph" (Ron Edwards), then "GM-Force" (Mike Holmes).