My understanding is that:
1. yer boat sinks.
2. At around 3m of water pressure the Hammar pops.
3. Providing you have followed the fitting instructions the raft floats to the surface.
4. Upon the painter being fully pulled (the painter length is usally marked on the raft) by the sinking boat the raft deploys, if your boats is in water shallower than the length of the painter you deploy it yourself.
5. If your boat sinks in water deeper than the painter then the bouyancy in the raft will break the "weak link" , if not depending on the sea conditions you either cut the painter or sit tied onto your wreck........handy for future diving
6. You sit and eat the Kendal mint cake hoping your insurance is up to date.
I'm not sure how all this would work on an inverted Rib. I don't think there would be enough water pressure to pop the hammar, so a bit of dooking would be required to set it free.