These devices help to prevent accidental inflation of manually activated inflatable PFDs, as well as automatic inflatable PFDs with manual back-up activators.
The pins (or clips, depending on the manufacturer) provide some resistance to pulling on the inflation handle to help reduce the possibility of accidental inflation should the handle inadvertently snag on some object. They will prevent most accidental inflations, since if the accidental pull is strong enough, it will break the pin, anyway.
The pins are broken when the PFD is inflated on purpose.
Regardless of the cause, when the PFD is inflated using the manual pull-handle, the pin must be replaced.
Always refer to the label inside your inflatable PFD to determine or confirm the type of mechanism used in your particular inflatable PFD.