This was pointed out to me on my very first day.
Shaking hands with the invisible man or
holding an invisible handbag
Also people using the plus/minus technique (rotating their shoulders into a + shape over the board, before swinging the board around back into a - shape.
In both cases it was pointed out that you can ride like this and some people can get down the hill just fine and plenty fast. I was shown a few people that were bombing down the hill just fine with both 'bad' techniques. (Once you see it, it's easy to recognise how untidy it looks). But that ultimately they'll hit a limit at some-point where it makes it hard for them to go further in their development.
If you're balanced and stable throughout a turn, then you can still use rotation etc to adjust if needed. But if you are off balance by default then anything unexpected and you're probably going to bite it.
That lesson has always stuck with me and I believe has had a positive effect on my own development.