In all seriousness though, I had heard that my whole life but never really knew how to implement it until someone explained it in this way. It's lifting with your hamstrings and glutes and upper back in reality, not just "legs"
I don't know how many men age 20-50 that I've seen joke about "lift with your legs!" and then proceed to laugh as the subject just fumbles around worse than they would have, because they're not sure what lifting with your legs means either.
The really bad part about all this, is that without good posture and defaulting to "cashew-style," you're preventing your lungs from using their full capacity. Which I think makes people slightly oxygen-deprived, more sleepy and daydreamy, causing them to sleepily lean forward into their screen instead of taking a minute or two and finding some good posture.
I don't know how you can fix this either. Most Americans do destroy their lower back and then spend their rest of their life with a lopsided body, because they have no idea what good posture is, and it takes a long long time to actually fix the part of your mind that causes bad posture. You have to be willing to accept that your entire way of operating your body has been slightly off, for a really long time.