So you have the average detective story, with a huge, widely spanning mystery that has both the detective, and the viewers stumped. You've got it going, but, now that you're in the thicket, you've run yourself into a corner. It would take more space than available to connect the pieces, and you don't want to drag the viewer along with boring step by step exposition, so what are you going to do?
Wait, your main character is a detective, isn't he? Why not just have him deduce that these things are connected, and move on. He's a genius, why not just leave it at that?
In short, when a characters makes a huge jump to reach a conclusion that has to be made in order for the plot to progress, but without any real explanation for what might have spurred the conclusion.
Often used in cases where the viewers already know that everything's connected and how they connect, but there's no in story explanation, and the plot really needs to get to the next part.
- Named for Batman, who, given his title as the World's Greatest Detective, can easily fall into this when a writer gets into a rut. The worst example of this may be in the movie Batman Forever, where he, after discovering that Edward Nygma was the one sending him creepy riddles anonymously (which to the untrained eye, probably looked stalkerish but harmless), instantly deduced that Nygma had actually killed a co-worker who was thought to have committed suicide. With no evidence whatsoever. Of course, this might have been set up by a psychiatrist who made a similar deduction earlier in the film, looking over the riddles and concluding that the sender was "obviously homicidal" (paraphrasing).