Halliday & Matthiessen (2014: 452):
Hypotaxis is the binding of elements of unequal status … The dominant element is free, but the dependent element is not.The hypotactic relation is logically (i) non-symmetrical and (ii) non-transitive. For example, ‘when’: (i) ‘I breathe when I sleep’ does not imply ‘I sleep when I breathe’; (ii) ‘I fret when I have to drive slowly’ and ‘I have to drive slowly when it’s been raining’ together do not imply ‘I fret when it’s been raining’.