Halliday & Matthiessen (2004: 505):
If it is a paratactic complex, this process consists of two happenings — two actions, events or whatever. If the verbal group complex is hypotactic, on the other hand, there is only one happening. Thus in a paratactic complex each verbal group has a definite voice, although the voice must be the same in each case; but in a hypotactic complex only the group that expresses the happening, the secondary group, actually embodies a feature of voice. The primary group is active in form, but there is no choice involved. (The exception to this is when the clause is causative.)