When guarding an airborne player, you have to be at the
point of contact BEFORE the player leaves the ground, even if that point of contact is directly backwards from where you were before the opponent left the ground.[/QUOTE]
You are not reading the rule correctly.
You would be incorrect giving a block to a player who is backing up, even if his new spot is two feet behind where he was standing prior to A1 "taking off" (Air Jordan!)