I found an explanation online (the link may not work outside of the States).
Quote:
Relative tenses represent deictic tenses in relation to other deictic tenses. (In McCawley 1971: 91, and Hornstein 1981: 120, the relation in question is syntactic subordination: in what McCoard (1978) calls the "embedded past" theory of the perfect, the present perfect derives from a past tense embedded under a present tense.) Thus had sung is the past-in-the-past, has sung the past-in-the-present, and will have sung the past-in-the-future. Similarly would sing is the future-in-the-past, is (about) to sing the future-in-the-present, and will be (about) to sing the future-in-the-future. Coincident (relatively present) tenses are ignored by many contemporary theorists, though Lo Cascio (1982: 42) writes of the imperfect, which is considered in traditional grammar a present-in-the-past, as a past coincident tense. (link)

Ceci n'est pas un seing.