Moving screen?

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I just wanted to add to Jurassic Referee's post:

If A2 gets ahead of B1, he may slow up and set a legal moving screen.

However, if A2 is ahead of B1 and then A2 changes path (cuts into B1's path), then A2 may still be guilty of a foul if contact occurs.

Also, if B1 were able to get ahead of A1, and then match paths (a legal "moving screen"), then B1 could slow up to try to hinder A1's drive to the basket.