I was reading about this on RSF and it wouldn’t have made sense to me before, but it certainly does now. Notice the great posture of the ladies in this photo and how relaxed they seem despite supporting heavy weight:
Women holding vases on head

Holding an object like that on the head leaves no choice but to stand up straight with nice relaxed posture. Any deviation such as slouching or leaning will make things difficult. I tried it with just a light book and sure enough, instantly I made a few helpful, automatic, small adjustments. I removed the book and the feeling was still there so that I only have to use “intent” to have the right feeling of a string pulling the head up gently.