Halliday & Matthiessen (2014: 503):
There is one further function of embedded clauses which is related to expansion in that, although there is no Head noun (so the embedded clause itself functions as ‘Head’), the embedded clause is the nominalisation of a process. For example, [[threatening people]] will get you nowhere. Such a clause is the name of an action, event or other phenomenon. It represents a ‘macro-phenomenon’ … ; let us call it an act.
An ‘act’ clause may also occur as Postmodifier to a Head noun of the appropriate class, for example the act [[= of threatening people]]. Hence it is reasonable to treat these as elaborations.