lol, you have to kick them to get them really tenderized first.
i suppose that since you'll be braising them for a long time, and they have a lot of connective tissue, you would bring the liquid to a boil (for various reasons: to boil off alcohol, to break down ingredients to empart their flavors, etc.).
from what i've cooked, you add something to a cold liquid then bring to a braise or simmer is when the protein is delicate, and/or is tender enough that you wouldn't want to destroy it or cause it to tighten up.
something like a flaky fish, or italian baccala that is simmered is started in a cooler/cold liquid or they'll fall apart or get tough, respectively.
hopefully, one of the pros will expand on this or correct me if i'm wrong.