Halliday & Matthiessen (2014: 305, 305n):
The verb may be unaccented as in a ‘relational’ clause or accented as in a ‘material’ one. The tense is also in a sense intermediate between that of ‘material’ clauses and that of ‘relational’ ones. When the Sayer is realised by a nominal group denoting a conscious speaker, the tense selection may be like that of a ‘material’ clause, with the simple present indicating habit or generalisation (i.e. an extended ‘now’) and the present in present indicating the narrower period of time; and the present in past often indicates simultaneity, just as it does with ‘material’ clauses. … The present can alternate with the past in conversational narratives just as it can with ‘material’ clauses …