Halliday & Matthiessen (1999: 217):
[Simple circumstances] are qualities — but construed not as qualities of a particular participant but as qualities of a figure as a whole […]. The usual function is as circumstance of Manner, with the meaning ‘in such a way’, ‘to such a degree’; and if the manner of doing determines the quality of the outcome there may be very little difference between a circumstance of this kind and a resultative Attribute: cf. don’t chop the parsley too fine / too finely.*
* Also, poetically, depictive Attribute: Do not go gentle into that goodnight.