good questions here, but matters little if you think about the across as deliberate or that it is a consequence of the swing...that is really the same. You deliberately swing so that the racket works across the ball, as a consequence of that swing. Your son is being coached wrong imo unless he is only hitting the last of the 3 balls. you can swing near straight to the last ball thru 3-5, but hit and work across the last one. I really don't like that teaching in any way I've seen it though, but just trying to make some sense of it.