Player with the ball turns his back and sets a screen

[christianH]

Hi guys

my english is very bad but I'll try anyway.

The playing situation is:

A1 has the ball and turns outside his cylinder to set a screen on B2 who gets stopped by the screen. A2 runs free towards the basket to get a pass from A1 ( the screener ).

At the time of the contact between B2 and A1 ( the screener) the latter was clearly moving ( turning outside his cylinder )

Here is the question: How can I judge that A1 was moving if his pivot foot was stationary?

Thanks for your help