I am not sure this is correct. If a basketball player left school at the end of the season to prepare for the NBA draft and did not officially withdraw, or was failing his Spring semester classes, that would bring our APR down. But as long as he was academically eligible when he withdrew, I don't think his leaving would hurt our APR.
That said, one player each semester being academically ineligible is enough to destroy a basketball team's APR.
From my reading, Iv20gt is closer to being right than your explanation. APR does not consider the academic eligibility when the S-A withdraws, but if they would be eligible to compete at the beginning of the next regular semester, e.g. the following Fall in the case of 1 and done basketball S-A. Because one of the requirements of NCAA eligibility is that you must pass at least 6 credit hours each semester, a 1 and done who withdraws during the semester will hurt the APR.
APR gives two types of credit: an eligibility point, and a retention point, and these are computed for each semester the S-A was on scholarship. So a typical S-A can "earn" 4 APR points each year. The eligibility point is earned based on the S-A meeting the academic requirements to be eligible the following full semester. The retention point is earned if the S-A is enrolled full-time 5 weeks into the following full semester, or if the S-A graduated before the following full semester. So a 1 and done player who withdraws before passing 6 credit hours in the second semester would earn only 2 out of 4 points.
This also puts this S-A in the category of an "0/2", meaning that they earned neither the eligibility point nor the retention point for a semester. This is a particularly damaging category, because is is the "0/2" S-A's combined with a 4 year APR below 930 which causes a loss of scholarships, with the number lost being equal to the number of "0/2" S-A's (up to 3).
On the other hand, if the 1 and done S-A remains in school through the second semester, passes 6 credit hours and meets all other eligibility requirements for the following semester, the school can request a waiver based on the S-A leaving to become a professional athlete. (They actually have to sign a contract; declaring for the draft or signing with an agent is not sufficient.) In this case, the second semester retention point would be removed from the calculations completely and the S-A would earn 3 out of 3 points for the year.
Disclosure: I am not in the business, and this is all based on my personal skimming of NCAA manuals and various write-ups - I freely admit the possibility of error. Also, there are other exceptions and nuances to the APR rules, and the procedures have changed some over the years.
