Halliday & Matthiessen (1999: 252-4):
Here the syndrome … occurs in a more general environment, that of construing a sequence on the model of a figure — grammatically, the sequence is construed not as a clause complex but as a clause.they shredded the documents before they departed for the airport => they shredded the documents before their departure for the airport
they shredded the documents before they departed for the airport => their shredding of the documents preceded their departure for the airportThese examples of a ‘sequence => figure’ metaphor both involve a sequence of the expansion type. However, this type of metaphoric shift also occurs with projection sequences; for example:The colonel declared his innocence.— where the congruent form would be a projection, either hypotactic or paratactic:The colonel declared that he was innocent.
The colonel declared, “I am innocent”.