When the meat is pounded, it becomes tender. Even people who don't usually cook can easily imagine it.
But why does hitting the muscle fibers and the connective tissue that holds them together soften them?
In fact, cutting the muscle fibers and connective tissue that way increases the water content of the meat by 5-15% after cooking. This is because cut muscle fibers are less likely to shrink, and damaged proteins in muscle fibers absorb moisture, resulting in juiciness.
It works particularly well with tough meats, and lean chicken breasts should be flattened by gently pounding them on a flat surface rather than the jagged surface of the meat tenderizer. Also, if the thickness is uneven, it will not cook evenly, so be careful. (Beat both sides of the meat evenly)