Since the heart is just a complex muscle, it’s the same principle as with your biceps or leg muscles. When you work out, the seperate muscle fibers get damaged and have little rips or are just exhausted. When you take a break, your body panics and takes action to help you heal as quickly as possible. In the process, the fibers are made extra strong and sometimes new ones are created. It’s kind of a “One step back, two ahead” thing.