When memorizing music--say a piano score--when I know it by heart, my mind no longer has to remember the written music. Something else kicks in--call it the old ticker. When you've got the music memorized and can play it by heart, you can really pour a lot of creativity into what you're playing because you completely focus on the sound and the touch.

I would guess knowing something by heart is knowing it so well that your emotions and creativity, once believed to have centers in the heart, are completely called into being, and your head, the great decoder, gets to take a rest.

...not that I really separate the head from the emotions, but am just thinking of the time when the expression "by heart" came to be.

Beating regards,
Wordwind