Ok ... my misunderstanding.

Just Another Ref:

With all due respect you can disagree all you want but every time you call this a block you will be wrong each and every time. As I stated in my post which you quoted, that for over sixty years the Rules Committees position has been: (a) A player who gains control of the ball must expect to be guarded from the moment he/she gains control of the ball; and (b) A player who does not have control of the ball has a reasonable expectation to not be guarded.

MTD, Sr.

Expectation of being guarded does not address whether or not the attempt at guarding is legally done. Upon further review of the OP, it says A1 lands then crashes into B1. This would indicate that B1 has legal position and is stationary, in which case PC would be the only call.

But I still find it conceivable that B1 could take a position which is not legal in the path of airborne A1 and A1 could contort his body in such a way that one foot might touch the floor before contact.

You are missing the point. The definition of guarding states that a defender does not have to give time and distance to obtain (NFHS)/establish (NCAA/FIBA) a Legal Guarding position against a player is in control of the ball and is not airborne. That means when A1 gains control of the ball while airborne, then returns to the floor just short of B1 and then charges into B1, A1 has committed a PCF. In the play being described that is exactly what happened.

While B1 took a position that was not legal if A1 had made contact with B1 before returning to the court, the instant A1 returned to the court before making contact with B1, B1's position on the court became legal. This is because the definition of the guarding was written from the belief that the player in control of the ball must be expected to be guarded at all times.

MTD, Sr.

P.S. Just remember, I am the possum that Gus would argue with, :p.

The only kind of position B1 might have that would still be a block would be one where B1's arms/legs/etc. were extended outside B1's frame, making it a block regardless of how or when A1 lands. If A1 contorts their body such that it permits them to land before contact, that is A1's problem.

Right. While unlikely, it is possible for the airborne player to touch the floor before contact and the call still be a block.

Nope, that's not what I wrote. Try reading my post again.

B1 takes a position in front of airborne A1, but his left foot is extended significantly in front of his right. A1 lands on his own left foot first followed by his right which lands on the foot of B1 causing both players to fall to the floor.

Can I penalize B1 for this?

If A1 were dribbling up the court and B1 took this same position and the same contact were to occur, would you penalize B1?

I don't see that it's possible to have the same contact on a player dribbling up the court as on an airborne player returning to the floor.

Time And Distance ...
 

Once the airborne player gains possession of the ball and returns to the floor the principles of legal screening go out the door and the principles of legal guarding come onto play, so time and distance also go out the door.