Well, I'll tell you what I know from my experience....
CD21 bw is supposed to check or confirm O in an "ideal" situation where you are expected to O on cd14. However, for most of us PCO girls, we rarely O on cd14 (don't get me wrong, some totally do and that is a blessing!); therefore, a cd21 check is pretty much useless -OR- shows either a very weak O or not at all.
The idea behind it is to check the progesterone level in your blood stream which should be elevated if you have ovulated, thus showing that the body (endometrial lining, Ph level, etc.) is preparing for implantation and viability of the (wonderfully blessed) fertilized egg (in the event that it occurs).
The problem with PCO girls is that many times we don't O until like cd18, 24, etc etc.... So a cd21 bw is useless. That is why, especially if you're using OPKs, fertility monitor, temping, etc. it's best to request that your bw be done at 7 dpo so that the progesterone levels will have risen and the bw should be more indicative of what's really going on.
Many times, you will find that (especially) general OB's will still want to do cd21 bw regardless of if you feel you've O'd late, I had an episode where I had a CBEFM confirm O on cd19 and the OB still insisted on doing the cd21 bw. Guess what, according to her I hadn't O'd. Ya think? Well I hadn't O'd 7 days ago! Does this make any sense??
I hope this helps and hasn't just confused you greatly!