Halliday & Matthiessen (1999: 286):
How do we show the agnation between congruent and metaphorical? There are really two distinct parts to this question. One is: do we need two separate phases in our representation? …
[This] raises the issue of metaphorical junction. In our interpretation, the text as it stands, with the grammatical metaphor left in, embodies semantic junction: it is not just a variant form, identical in meaning with its congruent agnate — it also incorporates semantic features from the categories that its own form would congruently construe. Thus engine failure is not synonymous with engines fail; it is both a figure consisting of participant ('engine') and process ('fail') and an element (participant) consisting of thing ('failure') + classifier ('engine'). In other words, we need both analyses in order to represent it adequately. This will always be true whenever the metaphor can be unpacked to yield a plausible more congruent form.