The Encyclopedic Dictionary of Applied Linguistics: A Handbook for Language Teaching doesn't even mention backshifting. Nor does the Longman Grammar of Spoken and Written English. Even the OED fails to mention backshift. The Longman Dictionary of Language Teaching and Applied Linguistics, Third Edition has no entry for backshifting, but under back-shift it instructs us to see direct speech. Unfortunately, this reference resolves to an entry that is entirely unhelpful on the matter. Apparently, this term is not widely used.
But do not despair. The Oxford English Grammar steps up to the plate. George Yule also talks about backshift in Explaining English Grammar, as does the Cambridge Grammar of the English Language. But these books are unanimous in defining and discussing backshift in relation to reported speech, and Firsten's topic has nothing to do with reported speech. Even Wikipedia concurs. Yule, for example, says:
"The change of forms, such as present tense (e.g., 'I am ill') to past tense (e.g., 'He said he was ill') in indirect speech."Terminology aside, the explanation offered strikes me as generally correct. But for those of us who are obsessed with categorizing grammar, it cries out for a label. The Cambridge Grammar of the English Language observes, "three uses of the preterite (past tense): past time, backshift, and modal remoteness." This last term, modal remoteness, is what we're looking at here. The question Is there something you want to see me about? is very immediate. It tramples social distance. This can make things more or less comfortable, depending on what each party gains or loses from recognizing social distance.
The point that tenses have uses beyond the signification of time is one that is not made often enough. Certainly, backshift, being one of those uses, is related to the topic at hand, but only remotely. There may actually be grammar references out there that use backshifting in the way Firsten does, but I've never come across them and some fairly anally retentive searching on my part has failed to turn up any.
[On an almost completely unrelated note, did you hear about the cosmologist who went to the costume party dressed all in red? He would move away from people and then ask them what he was. Answer: redshift]