It would need extensive testing. If anyone has messed with the laser guns the police use to detect speed, it is very easy to fudge the results. All you have to do is be moving the gun in the direction of the vehicle when pulling the trigger and you can add 5-10km/h to it. Bear in mind, the last time I fucked with this was 20 years ago, the guns these days probably have gyros that account for this, BUT in the case of passing the ball at speed, the same rule would apply. If someone is running with the ball, if they worked out a smooth way of initially moving the hands forward (with the ball) to give the sensors an inflated "running" speed, then the pass would be allowed to travel a lot further forward (and still be detected as "backwards out of the hands") than their actual running speed should allow.
I have made my opinion on this known in the past. I don't like the "backwards out of the hands" rule. The same is not applied to knock-ons, you can't be running for the ball, accidentally brush it with your fingers in a way that makes it travel forwards, but not as far forward as the player running (which is actually more common as you think, think of a glancing brush of the ball as players compete for a bouncing ball). The marker they use on the video ref with these is if it travels in a forward direction in relation to the field (not the player), it is a knock-on.
By the "out of the hands backwards" rule, if a player is running at a decent speed (let's say 25mk/h, which is 7 metres/second), if they throw a pass that takes 1 second to hot the mark, it can travel up to 7 metres forward (on the filed) and still be considered flat or backwards. To see that happen would not pass the sniff test to viewers, so I don't think the "out of the hands" is going to be an acceptable marker to viewers. It should be "forward in relation to the field" only IMO.