it does exactly what it says on the tin (idea)

Useful phrase originating in a series of infuriating advertisements for Ronseal varnishes. To paraphrase the ad:

Ad man: Ronseal quick-drying five-year woodstain is a quick-drying woodstain from Ronseal, that lasts for five years. Ronseal. It does exactly what it says on the tin.

Although "Does exactly what it says on the tin" is a registered trademark of Ronseal, it is widely used to indicate self-evidence or redundancy. e.g. "What does the self-destruct system do?", "It does exactly what it says on the tin".