This has to be one of the most pernicious pieces of postural advice out there, it's unhelpful and needs to be burst once and for all. Given how ineffective it is it's amazing that it's been so durable.
It also raises the question of how effective imagery is in general as you never know how someone is going to filter it. What they're going to do with it. F.M.Alexander was against the use of imagery for this reason. Not being one to tell people what they should do in general, even though I don't offer imagery in my teaching, I don't discourage those who feel comfortable using it. But I do highlight the potential pitfalls and to be mindful of them. An image is only as good as it's ability to positively improve behaviour, it has no inherent value.
I challenge you, have you ever maintained this advice for even half a minute?
So the next time someone offers you this image, imagine me next to you with a pin!
If you have any questions about how the Alexander Technique could help you, please do get in touch, I'm happy to answer any queries you may have.