"Post-Nut Clarity": What's Actually Happening

Orgasm triggers a release of several hormones and neurochemicals, notably prolactin, along with a drop in dopamine and testosterone in a lot of cases (particularly for people with a penis, post-ejaculation). Prolactin in particular is associated with the sense of satiation and reduced sexual motivation that follows orgasm — part of what's sometimes called the refractory period. The shift from a dopamine-driven, motivated state to a more settled, prolactin-influenced state is a plausible physiological basis for the "sudden clarity" a lot of people describe.

Not consistently — this is a real hormonal shift, but the subjective experience of "clarity" varies a lot person to person and situation to situation, and there isn't a large body of rigorous research specifically studying the subjective psychological experience under this specific name. It's best understood as a real, biologically plausible phenomenon with an informal name, rather than a scientifically established, universal effect.