Halliday & Matthiessen (2014: 196-7):
There is one other element that occurs in major clauses but which can also function on its own in dialogue. This is a textual element – the Continuative, which is used to indicate how the clause relates to the preceding move in a dialogue: well, oh, yes, no, and so on. Such items can also function on their own in dialogue, indicating that the listener is tracking the current speaker’s contribution. This has been called ‘backchannelling’ … Such minor clauses include yes, mmh, aha, sure. They do not constitute a turn in their own right; rather they serve to ensure the continuity of the interaction by supporting the current speaker’s turn … In face-to-face conversation, they may of course be accompanied — or even replaced — by other, ‘paralinguistic’, indicators such as nodding.
Note that a function of so-called High Rising Terminal tone is clearly to ‘demand’ the polarity supplied by ‘backchannelling’.