[a1369209993]

A less eloquent but probably more helpful translation/phasing might be:

You have not truely mastered something until you've forgotten the step-by-step process of it (and presumably had to reconstruct it by doing it and paying attention to how you do so).

(This is also why it can be hard to teach something you're very good at - often, you literally don't know (well, remember) "how to do it" in a form that's actually useful to them.)