Halliday & Matthiessen (2014: 165):
…the unmarked positive has no Mood element, the verb form (eg look) is Predicator only, with no Finite in it. The other forms have a Mood element; this consists of Subject only (you), Finite only (do, don’t), or Finite followed by Subject. Any of these can be followed by a Mood tag: won’t you?, will you? — showing that the clause is finite, even though the verb is non-finite (the imperative of be is be, as in Be quiet!, not the finite form are). Historically the forms do, don’t derived from non-finite forms of the verb do, but they now function analogously to the Finite operator in an indicative clause; compare the dialogic sequence Look! – Shall I? – Yes, do! or No, don’t!, with the response consisting of Mood element only.