Halliday & Matthiessen (1999: 258-9):
The overall effect of the grammatical metaphor is that semantic relations between one element and another, and between one figure and another, become progressively less explicit as the degree of metaphoricity increases. We can illustrate this by taking a text example and relating it to more congruent and more metaphorical variants:
Notice how the semantic information construed by the grammar in the most congruent version is gradually lost at each step in the course of metaphoric rewording.