One look at these two pictures and you instantly see what a comfortable, upright posture is and isn’t.
The girl on the left is upright and relaxed. Her bones are stacked, eliminating potentially harmful stress on her spine. She is sitting in the center of her pelvis. The is the pubic bone area, we usually call it the crotch At first when you practice this you may feel more ”tipped forward” than usual. There will be a soft arch in your back but it will feel much more like a big arch because it is so new to your body. As you relax into this posture your spine will balance over your pelvis — which means less “working” of the muscles to hold you up.
The girl on the right has a tucked buttocks which automatically causes her head and shoulders to come forward. The crossing of her legs keeps her stable and from rolling backwards but, over time, could create numerous hip and spine problems.