A brined chicken thigh and an unbrined one can hit the exact same internal temperature and still come out of the pan holding very different amounts of water. Salt is why — it dissolves part of the muscle's myosin lattice, the structure swells open, and the water already inside grips more stubbornly once heat starts trying to drive it out.
Salt dissolves part of the muscle, and that's the whole trick
Muscle fiber is mostly a tight lattice of two proteins, actin and myosin, packed close enough that there's little room between them for water to sit. Salt breaks that up. At the concentrations a brine runs at — anywhere from about half a percent to six percent of the meat's weight — sodium and chloride ions pull some of the myosin filaments apart and partly dissolve them, and the lattice swells open. More space appears between the filaments than was there in raw, unsalted muscle.
Water fills that space. Not by soaking in from outside the way a sponge takes up a spill. Some of it is water the meat already had, some is water a wet brine adds, and both end up held inside a structure that's now physically more open and chemically more attracted to water than it was before salt touched it. Brining doesn't pack extra liquid into meat. It rebuilds the muscle so the liquid already there — or added by the brine — has somewhere to stay.
Why that shows up as "juicier at the same doneness"
Cooking does roughly the opposite of brining to that lattice. Heat makes the same proteins contract and clamp tight again, and normally that contraction pushes water out — most of what's pooling under a rested, unbrined steak got squeezed there by exactly this. Salted muscle contracts less severely, because the swollen, partly dissolved structure has already lost some of its grip strength. Two pieces of chicken pulled from the oven at the identical 165°F (74°C) don't lose the identical amount of water. The brined one holds more of it at that same internal temperature, because the protein doing the squeezing has changed.
Why concentration and time trade off against each other
Salt doesn't teleport into meat. It diffuses in, moving down a concentration gradient from the salty exterior toward the unsalted interior, and diffusion has a predictable relationship with both strength and distance. A stronger brine builds a steeper gradient and pushes salt inward faster — a concentrated dry-brine puddle sitting right on the surface can finish reabsorbing into a thin chop in under an hour. Distance matters more than strength, though. Diffusion time scales with roughly the square of how far the salt has to travel, not the distance itself. Double the thickness of a cut and the center needs about four times as long to reach the same salt level.
Telling it's working without a thermometer
None of this needs measuring to track. Early on, the surface beads with drawn-out moisture — that's osmosis running before diffusion has caught up, visible as a cloudy pool in a wet brine or a slick sheen on a dry-brined roast. Press the meat and it should start to feel slightly firmer and spring back rather than stay dented, a sign the protein has begun to swell and reorganize. Smell it. Raw meat smell only. A sour or sharp edge means the process ran too long at too warm a temperature, not that it worked harder.
The one place this becomes a safety question, not a texture one
Leaving meat in a brine too long is a texture problem. The same swelling that makes meat juicier keeps going past its useful point, and the muscle turns dense and springy, closer to ham than roast. Leaving raw meat sitting in liquid or a salty surface film at room temperature for hours is a different problem entirely — that's the temperature band bacteria grow fastest in, and it doesn't care how well the salt is doing its job. Time works against texture. Temperature works against safety. Keep the brine refrigerated and the second problem never comes up.