It's good that there's some research into this - to confirm what is generally obvious to anyone who studied anything. You have to think about what you are doing, write things by hand, use the skill to improve and retain it.

Common example here is learning a language. Say, you learn French or Spanish throughout your school years or on Duolingo. But unless you're lucky enough to be amazing with language skills, if you don't actually use it, you will hit a wall eventually. And similarly if you stop using language that you already know - it will slowly degrade over time.