This article is from the Piano Playing from Memory FAQ, by Isako Hoshino rmmpfaq@yahoo.com with numerous contributions by others.
After a piece has become automatic, you can lose your theoretical
memory of it just like you can forget how to explain how you tie
your shoelaces. When you are playing up to speed, there is not
enough time to think in words about all of what you are playing.
Slow practice is one way to allow you to bring back and
consciously apply your theoretical knowledge. Maintaining your
ability to think of the piece in terms of the identity of its
notes and structures will in turn assist you in recovering from a
memory lapse.
Likewise, if you use your visual memory of the score to provide
cues, then, as your automatic memory becomes solid and your
playing has become independent of the score, you may need to
maintain your visual memory.
 
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