When you start running, here’s what happens: Nothing.
You run and run, burning lots of calories, and yet your weight remains the same and your stomach looks the same.
You have to stick with it, and if you do, eventually your body decides that no, you aren’t kidding, and yes, it has to do something different. So finally it turns on your metabolism and now your weight starts to come down.
When you return to the chessboard after a layoff, you start doing tactical puzzles. You do 100s of them and still nothing happens. And you throw in blindfold games and other mental exercises. And then eventually, abruptly even, your rating starts going up.
It’s not that you suddenly know more about the game. It’s more like you’ve finally convinced your brain that no, you aren’t kidding and yes, it has to re-fire some synapses and get itself in gear.