Halliday & Matthiessen (2014: 569-70):
Here the verb in the primary group is a very general one of the ‘intensive: ascriptive’ class, and it is elaborated by the verb in the secondary verbal group. The semantic relation between the two is one of PHASE (see Table 8-3). The basic notion is ‘be (intensive) + do’, using ‘do’ to stand for any process. The two dimensions of phase are time-phase and reality-phase.