guarding the

[blindzebra]

First we should not assume the intent of the offensive player when they leave the floor.

There needs to be a distinction between a dribbler and an airborne player who gathers the dribble and is entitled to land.

You need to handle this like a player catching a pass while airborne, see CB 10.6.1.B. If B1 moves into the spot after A1 has jumped any contact before A1 lands is a blocking foul.

To get to ysongs question.

If B1 is on the spot before A1 leaves the floor to pass or shoot you have either a player control foul on a try or a team control foul on a pass.

If A1 is airborne on a try or pass and B1 then moves to the spot it's a blocking foul.

If A1 jumps and gathers his dribble, then B1 moves to the spot, if contact occurs before A1 lands it is a block, if A1 lands and then contact occurs it is a PC foul.