The flashpoint of a story is its first turning point and has the function of bringing chaos where there was order.
It is the point of no return, the trigger that gives rise to conflict and action. All in all: it is the event that provides the writer with a story to tell. Here you have some examples:
The Wizard of Oz: the flashpoint is the tornado that transports Dorothy from Kansas to Oz.
Casablanca: Ugarte decides to hand Rick the envelope with the letters of transit.
Star Wars: Darth Vader attacks Princess Leia’s spaceship.
In the latter case (Star Wars), the flashpoint is not visible to the spectator, as it occurs before the first scene of the film. This also happens in many detective stories, such as those of Sherlock Holmes. The critical moment (either a robbery or a murder) is usually prior to the beginning of the story.
When should it happen?
As you have seen in the examples, the flashpoint lacks a fixed position. In some cases, it can occur before the story begins; in others, it happens after the introduction, because the author has decided to present the characters and their universe first.
Which of these options is the best? It depends. Sometimes, it will be more interesting to get to the point so that your readers (or spectators) get hooked to the story from the beginning. Other times, it will be more advisable that you first focus on the context surrounding the story so that those reading it identify themselves with the protagonist and understand better the importance of the flashpoint.
In any case, this should take place before the end of the first quarter of the story, or you would be writing too long an introduction. Be careful with that, as it might bore your readers.
How do you know you have chosen a good flashpoint?
This may seem like the million dollar question, but it isn’t so. There are a number of aspects you can consider in order to know whether or not you have chosen a good flashpoint.
Change. It must introduce some variation in the plot, alter in some way the life of the characters and do it permanently, with no turning back.
Conflict. It must bring about some type of conflict between the characters.
Action. Or, better said, reaction. The crucial moment must have consequences; it must provoke action and make things happen.
Suspense. If you take into account the three aspects mentioned above, your flashpoint will create suspense and intrigue. Besides, it will encourage your readers to ask themselves how the story continues, and that will keep them reading.