[drothamel]

If the defender obtained legal guarding position, then it is either a player control foul, or a travel, if you judge the contact to be incidental. A player in posession of the ball is entitled to neither time, nor distance, and must expect to be guarded.

Now, if A1 jumped to make the catch, then B1 must have obtained her legal guarding position prior to A1 leaving the floor. She can't wait until the girl is in the air and then show up underneath her. Same principle as a player taking a shot.

The play that you describe is covered in the case book somewhere, but I don't have it with me.