Halliday & Matthiessen (1999: 174):
This category lies at the borderline of elaborating and extending: compare he spoke like an expert ‘in the manner of’, he spoke as an expert ‘in the rôle of’. The analogous participant is that of Attribute in an ascriptive clause as in he was/seemed an expert, which is construed as elaborating; but the analogous sequence is an enhancing clause complex he spoke as if he was an expert.