Transitive: to open (part of one's clothing), to bare/expose (e.g., one's chest) — to cause clothes to come open.
彼は暑さで襟元をはだけた。
He opened his collar (はだけた) because of the heat.
Intransitive: (of clothing) to come open, to gape, or to become exposed (e.g., clothing blowing open).
強い風でコートがはだけてしまった。
The strong wind blew my coat open.
Archaic transitive: to open wide or spread apart — used historically for opening legs, eyes, mouth, etc.
古い文献には、驚いて口をはだけるという表現が見られる。
In old writings one finds expressions like 'to open one's mouth wide in surprise' using はだける.