A reader's comment (handle: khawaga) to the Language Hat:
There's a wonderful anecdote from the Arabic tradition about Abu Nawas (famed 8th century poet) that gets to the heart of what I think the memorization of poetry is about. Early in his career Abu Nawas seeks out Khalaf al-Ahmar, a famed critic, asking for his authorization to compose poetry. Khalaf tells him he won't give it to him until Abu Nawas memorizes 1,000 selections of classical Arabic poetry. Abu Nawas disappears for a spell then comes back and tells him he's memorized the poems and spends several days reciting them all to Khalaf to prove it. Then he asks for his authorization.
But Khalaf tells him he won't give it to him unless he now goes and forgets all 1,000 poems "as if you had never memorized them." Abu Nawas tells him that will be quite difficult, but Khalaf insists so Abu Nawas disappears again and later comes back and tells him he's forgotten them all, "as if I'd never memorized them," whereupon Khalaf tells him that now he can go and compose his own poetry.
The "forgetting" here is a lovely metaphor, I think, for the difference between reading or knowing the words of a poem and actually "holding it in your head" as LH said. The 1,000 poems were not actually erased from Abu Nawas' memory so much as they were fully internalized, to the point that it was, in fact, as if he had never memorized them, but had instead been born with an innate knowledge of an entire poetic tradition--its meters, rhythms, diction, vocabulary, etc. It's the difference between knowing a rule of grammar for some language, say, and using the rule without thinking when you speak.