Halliday & Matthiessen (1999: 271):
Thus grammatical metaphor is a means of having things both ways. An element that is transcategorised loses its original status because of the nature of the semantic feature(s) with which it comes to be combined … A[n] element that is metaphorised does not lose its original status. Its construction is not triggered by its being associated with any new semantic feature. If it has a new semantic feature this is a result of the metaphorising process. … It has become a ‘junctional’ construct, combining two of the basic properties that the grammar evolved as it grew into a theory of experience.
Halliday & Matthiessen (1999: 283):
… we have shown that the metaphorical version is not simply a meaningless (ie synonymous) variant of some congruent form; it is ‘junctional’ — that is, it embodies semantic features deriving from its own lexicogrammatical properties.