Halliday & Matthiesen (2014: 315):
The location of circumstances on this cline will determine patterns of agnation. For example, around (1), Process with lexically incorporated feature of manner: sway can be glossed as Process + Manner, ‘move slowly or rhythmically’, and wobble as ‘move unsteadily from side to side’; but around (2), Cause: because of his asthma is agnate with either a participant, e.g. asthma prevented Cam from sleeping or to an enhancing clause of cause, e.g. Cam could not sleep because he had asthma.