Anacoluthon

Anacoluthon is a break in the grammatical structure of a sentence, where a construction is abandoned partway through and replaced by a different one, leaving the first part unfinished.

I told him the whole thing was — well, you were there.

The sentence sets out as a statement, runs into something the speaker will not say, and restarts as a different sentence. Nothing completes, and the reader understands perfectly.

This is how speech works. People begin sentences before they know how they will end, change their minds mid-clause, and abandon constructions they have got themselves into. Dialogue written without any anacoluthon reads as drafted rather than spoken, which is one of the quieter causes of on-the-nose dialogue.

Two uses on the page.

Characterisation in the moment. A speaker who breaks off is a speaker under pressure, and the break is more convincing than a line of narration reporting that they were upset.

Interior collapse. In free indirect discourse, a broken construction puts the reader inside a thought that has failed to complete — And if the letter had come a week earlier, if anyone had thought to — no.

The risk is obvious. A deliberate break must be legible as a break, or a copy editor will fix it and a reader will read it as a mistake. The dash is the usual signal. Used sparingly, it is voice; used everywhere, the prose simply cannot finish a sentence.