Halliday & Matthiessen (2014: 482):
Hypotactically enhancing clauses may be finite or non-finite. The finite ones are introduced by a binder (‘subordinating conjunction’). The non-finite are introduced either (a) by a preposition such as on, with, by functioning conjunctively – note that sometimes the same word is both conjunction and conjunctive preposition, e.g. before, after; or (b) by one of a subset of the binders – there are a few of these, such as when, which can function also with a non-finite clause.