A slightly different scenario:

B2 is guarding A2. B1 is guarding A1. A1 drives past B1. At what point would A2's "boxing out" (really screening) of B2 constitute an illegal screen? What determines that B2 is no longer guarding A2, but is attempting to guard A1? Is this strictly the official's judgment or are there guidelines for this?

I do not have an easy answer for you. I think you just have to see the play enough to know. I will say this, I will and have called fouls when the offensive player or screener starts using their arms and hips to keep them away from the ball handler as they go to the basket. A lot of people allow this to take place without any penalty. I think you have to use your best judgment. You know when the offensive player without the ball is trying to move to give space if you see it enough.

Peace

Don't know if there's any specific guidelines I could point you too, however, the minute A2 moves his position (LGP) while B2 is trying to defend the basket and guard A1 and contacts B2 preventing him from gaurding the person with the ball. That's illegal.

If A2 is running in the same direction as A1, then it's tough to determine but if we don't have a break away layup type situation, the offensive players needs to remain still. I will also offer a couple of other points to consider. If the defender has no possible chance of preventing the score or play, I'm not calling an illegal screen unless of course there is blood or an intentional foul. What has not been discussed here is time and distance. If the offense has gone into their shooting motion and release the ball, and a secondary defender is fouled by a secondary offender (A2), then you can count the bucket and still have a foul on the offense. TC rules would then come into play and you would administer as such.

I don't know that it matters if B2 is "no longer guarding A2". If A2 was managing to screen B2, and then A2 starts to move around B2, and A2 chucks him or bumps him, well, that's illegal,....regardless of whether B2 or B1 is gurading or screening A1 or A2.

If B2 gets into the path of A1 legally before there is contact, then he can draw a charge. If he tries to get into the path and doesn't make it, then it's a block. Same as always.

Regardless of who's guarding or screening whom, you just apply the rules as you've studied them, and that helps you determine who's legal and who's not.

Or am I just misunderstanding the question?

Jurassic, does this reach a new peak of bizarre? I don't know if there could be more obfuscation in 200 words if you said it yourself!!

Lol, I read that a coupla hours ago and it wasn't even worth the usual WTF.

Ols School speaks a strange but oddly beautiful language, one that has not been interpreted into other earthly languages yet. It is only understood by himself and a few other denizens of the planet that he dwells on. The only thing that we know for certain is that his strange language has got absolutely nothing to do with basketball officiating.

Naw, you got 'er right. :)

Do you listen to Car Talk? Are you familiar with the "Andy letter"? Referenced OS post brings the Andy Letter to mind.

There will be abosolutely NO obfuscating on this website. This isn't one of those sites for crying out loud.:D

Or you'll go blind!
http://www.ncaauofficials.com/images/referee_blind.gif

Dude, you should have stopped here.
Really?Oh, we wish you wouldn't. That's because the discussion was "if" a foul could occur on the offense in this situation. It can, so then we could move on to T&D if we want to determine whether an illegal screen is involved. Does not matter if it's a secondary defender or not. Once the shot is released, only an airborne shooter is still considered "offense." Therefore, TC rules would not come into play (when the foul is committed after the shot is released) unless the foul is committed by the airborne shooter.
I'm not sure if you're wrong, unclear, or both.