You know what I was always confused about? You're supposed to keep an arch in your back, right? Well, I can keep one until I break about five inches below parallel - then I can no longer feel the gap in my erectors. I'm guessing this is slightly rounding - my tailbone isn't a clear, straight shot up to the back of my neck, ya' know?
So...is this something to be avoiding? Is it due to lack of flexiblity? I can't see how since I can fall into a split, but...I don't want to fuck up my back, and lately it has been hurting.