The Big Screw is a turning point in which the story itself turns against the heroes. The heroes will be confronted with a situation in which something tries to go really wrong. Usually, it’s something they trusted or something they were counting on. A major NPC betrays them or leads them into a trap. The villagers give up hope and refuse to fight the big bad’s army. Now, the heroes might succeed against the Big Screw. The trap might fail. The heroes might kill the traitor. Or catch him just before the betrayal. Or the heroes might give a rousing speech and turn the villagers around. Unlike the Major Setback in Fields’ structure, the Big Screw is a little more open-ended. If the outcome is bad, the heroes are at their lowest point and have to resolve to fight anyway. If the outcome is good, the heroes have been tested and found worthy of facing the climax. So, it’s fine either way.
For the Big Screw to have any power, though, it has to be an emotional thing. A traitor doesn’t hurt unless that traitor was someone who you genuinely cared about and who you believed would never betray you. A random stranger being kidnapped is just another NPC victim. Even a family member being kidnapped isn’t terribly moving unless you had a fight with that family member earlier and might never have a chance to make amends. A turning point is only as strong as the emotions it evokes. And this one has to evoke feelings of desperation, hopelessness, and/or extreme resolve.
After the Big Screw, there shouldn’t be any other further challenges or obstacles before the Climax, but there might be a scene or two that gives the players the opportunity to interact with each other, to react to the screw, to recommit themselves, pledge their support, resolve any lingering personal plot issues, and to plan their assault. Ideally, the Big Screw should come just as the heroes feel ready for the Climax, so there shouldn’t be anything left to do – story-wise – except clean up the emotional mess the Big Screw leaves behind.