Halliday & Matthiessen (2014: 550-1, 552):
Let us return briefly to thematic considerations within a hypotactic clause nexus. Compare the following two hypotactic nexuses from a procedural text:
α ^ β fry the onions until slightly brown
β ^ α if you want a more substantial stuffing add a little mashed potato
In the first nexus, the sequence is α ^ β (progressive sequence). Here the dependent clause is given rhematic status. In the procedure, the process of frying the onions is started before the change of colour takes place: the sequence of clauses is iconic with the sequence of events. In the second nexus, the sequence is β ^ α (regressive sequence). Here the dependent clause is given thematic status: see the first structural layer (Theme₁ ^ Rheme₁) in Figure 7-27. This thematic clause signals a break in the procedure and introduces a variation on the basic method. This re-orientation in the development of the text is achieved by giving the conditional dependent clause thematic status.
