Re: Kamae vs Shizentai (hips underneath)
I'm going to note before chiming in that I'm really a beginner compared to pretty much everyone else here, I don't know much about anatomy, and I reserve the right to be completely wrong.
But as far as I can tell, when standing with the feet side-by-side, say shoulder or hip width apart, when I tuck my sacrum underneath and relax the hips, making them level with the ground, my back straightens up, my head rests much more comfortably on my shoulders, and my knees naturally turn to be in line with my feet, instead of turning in as they usually do. This is what I was taught in Tai Chi and have read in numerous books by people that supposedly really know what they're talking about.
When one foot is in front of the other, as they are in a hamni stance, my hips tend to want to rock forward a bit and my butt tends to stick out a little but I try to not let this happen. I find that keeping the sacrum tucked helps me initiate movement with my hips and shift weight easily from one leg to the other. It is harder to spread my legs wider apart and take big steps while doing this but over time I've been able to make them longer and longer as my hips progressively stretch and relax.