If they are cramming Biden for the debates trying to get him to memorize his talking points, then they are probably canceling public appearances because that will be extra stimuli that might confuse him and cause him to forget his memorized lines.
He's probably getting flash card treatment, drilling him over and over on the questions and answers so they become automatic responses, but this will probably only last for a few days before he begins to lose the recall.
Public appearances with non-debate related stimuli might disrupt this pattern of question and recall.
What will happen during the debate is that Biden will start out strong, but as President Trump keeps bringing up different subjects, Biden will start to forget his lines; only the "top" lines, or his favorite one, will be remembered. After about an hour, Biden will run out of things to say and he will begin repeating the same few lines over and over.
President Trump will eventually close by saying that he's fluent on any topic that's asked of him, and then he will ask the viewers to compare him to Biden's performance tonight.
-PJ
He will “test positive for COVID” 3 days before the debates and it will take “some time” to recover. That is all.