Why Heston Blumenthal says it needs to soak chicken before low-temperature baking in salty solution of water? I always thought that osmosis goes better if water would be unsalted, and meat would be more tender soaked in pure water?
What you are referring to is called a brine. It's pretty common to brine meats, especially poultry (think Turkey, etc). Brining meat before cooking helps break down protein bonds, and create a more tender, moist finished product.
thekitchn.com has a very succinct explanation:
What Is Brine?
Brine is a salt solution made by mixing salt and water, usually about 5 to 8 percent salt by weight. Some recipes include sugar and other ingredients to add flavor to the meat being brined, but a basic brine is a salt-water solution.
How Does Brining Work?
Here are three major functions accomplished by brining — and reasons to try it. It's so easy, too.
Meat absorbs some of the liquid: When a piece of meat is soaked in a brine solution, that solution is slowly drawn into the meat, and even though some of it is inevitably lost during cooking, it still makes a big difference. Since the meat starts out with more liquid within, it ends up juicier and more moist when cooked.
Muscle fibers are dissolved: Highly concentrated salt solutions will cause proteins to precipitate (essentially forcing them to aggregate with each other and clump together). On the other hand, a low-concentration salt solution has the opposite effect and actually can increase protein solubility and allow more proteins to dissolve. So brine actually helps dissolve some of the muscle fibers, which helps to reduce the toughness of meat.
Muscle fibers and meat proteins denature: A salt solution can denature proteins, essentially unfolding and unravelling them. As they unfold, water works its way in between these proteins so there is more water in between the meat proteins as the meat cooks. This results in a more tender cooked meat.