From life experience.
When a student does not understand what the teacher is saying, when the ‘import’ does not sink in, no matter how many times he may recall the teacher’s words afterwards, even if he took notes while the teacher was explaining, he will not understand. No amount of memory recall of the teacher’s words will be of any use.
When a student does understand, that understanding will be his; it will not leave him. The teacher’s words may well be forgotten, or they may not, it does not matter.
In short, once understood, recollection of the lesson is not necessary; if not understood, it would be futile.
I hope seeing it from the human level simplifies things somewhat, Ikoren,
may you progress on the Path.