Halliday & Matthiessen (1999: 10):
… clauses that are shifted downwards on the rank scale to serve as if they were groups or phrases. Such down-ranking is known as rank-shift. This has the powerful effect of expanding the resources of grammar by allowing the meaning potential of a higher-ranking unit to enrich that of a unit of a lower rank. … Such rank-shifted clauses construe what we call macro-phenomena.