As I naively understand it, that's when we do or say something then narrate ourselves why we decided to do so. We think non-verbally, then verbalize a plausible rationale for it, post hoc.

I'm not sure that applies to discursive writing, when we essentially use rules of logic to decide on the course of the narrative. Non-verbal heuristics still applies, of course, but we constrain it, so it's probably not entirely post hoc.