Flash Cut

A frame or two of something else appears and is gone before the audience can name it. They register that something happened without being able to say what.

That is why it reads as memory, premonition or intrusion. A shot held long enough to study belongs to the scene. A shot that vanishes belongs to somebody's head.

Duration is the whole technique. Two to six frames at twenty four frames a second, which is under a quarter of a second. Longer than that and the audience reads the image properly, which turns it into an ordinary insert.

Make the flashed image simple and high contrast. At this length a busy frame registers as a grey smear, so one shape, one face or one strong colour is all that survives.

Generate the flashed image as its own clip and cut a handful of frames out of it. A model has no control at this resolution, so the frame count is set in the edit.

Use it for a memory breaking into the present, for a warning the character cannot explain, and to seed an image the film will pay off later at full length.

Avoid it when the audience needs to understand what they saw. Information that matters belongs in a shot they can read.

A crash cut is abrupt and then stays on screen. A flash cut is gone. If the incoming shot plays out, it is not a flash.

An insert also drops a detail in, and an insert is held long enough to be read. Flash cut trades legibility for the feeling of having almost seen something.

Denis Villeneuve threads brief glimpses of a child through Arrival (2016), cut short enough that the audience files them as memory. The film depends on that reading holding until the end.

The model returns a held shot, because a clip has a minimum length and a few frames is not a clip. Generate the image, then cut the frames you need out of it on a timeline.

The second failure is a flashed image nobody can parse. Ask for one simple high contrast subject on a plain field, and skip any prompt that describes a scene with several elements.