All they need to measure is the first few cm's out of the hand, what happens after that is irrelevant. The ball can travel forwards, backwards, sideways, float, wherever it likes, just as long as it leaves the hand/s backwards for 1 mm or more. That's the difficulty, the measurements is over a very short distance and at reasonable speed, the passing player is moving, plus the ball is often rotating and/or spinning as it leaves the hand. It's hell of a thing to measure.
If it was easy it would have already been done.
Always a Bulldog