As a kid I did a thing with my fingers where I'd start with this simple pattern (mapped to some finger motions):

DOWN UP UP DOWN

This is the UP-pattern. The DOWN-pattern is UP DOWN DOWN UP.

Then I'd do it again, but replacing every DOWN with the DOWN-pattern and every UP with the UP-pattern.

Then I'd try to do it again for that pattern. I don't remember how deep I could get. It wasn't very far! But I loved doing it.

Decades later I met someone who had done the exact same thing as a kid, though he called it LEFT RIGHT and moved his tongue around to do it, rather than tapping it out with his fingers.

Technically I ALSO did it left to right, I just didn't think of it that way. For UP-pattern I'd tap, or really roll/drum, left-to-right, index-middle-ring-pinky. And DOWN-pattern was tapped right to left.