Bring it to a bare simmer: small bubbles breaking the surface, not a rolling boil.
Simmer is the most abused word in recipe writing. Most people read it as boil a bit less, and a pot at a rolling boil looks like it's working harder, so it feels like the right answer.
What a hard boil actually does
It churns. In a stock, that agitation drags rendered fat and fine protein back through the liquid and emulsifies them into it, which is how a clear stock turns cloudy and greasy. You can pull it back with an egg-white raft, but it costs you an hour and some body, so it's far easier not to do it.
In a braise it's simply too hot. A bare simmer sits around 185 to 195°F. A boil is 212°F. Collagen keeps contracting as the temperature climbs, and past about 185°F the meat starts drying out and going stringy, so the same cut comes out worse from the hotter pot. Long and gentle isn't a stylistic preference, it's the difference between the two textures.
The picture to look for
A few small bubbles breaking somewhere on the surface. Not a bed of them, not a churn. If the surface is moving all over, it's too hot.
Where a rolling boil is right
Pasta, blanching, and sugar work. Pasta needs the churn to keep from sticking and to work the starch loose. Blanching wants the shortest possible time in the water. Sugar has its own temperature targets entirely.
And boiling isn't a mistake, it's a choice about the liquid you want. Tonkotsu, Chinese white stock and bouillabaisse all boil hard on purpose, for exactly the reason a consommé must not. Boiling emulsifies. If you want a clear broth, don't boil it. If you want a milky, bound, unctuous one, boil it hard.