Getting the butt out of the water

I was watching some film of elite swimmers, and one constant I see is that they all have part of their butt sticking out of the water in the freestyle. In fact, even the good young swimmers I see at my kids' meets do the same. In other words, they ride so high in the water. It's one thing to keep the hips up by pressing down with the chest, but it seems to be another thing to get your whole body to rise that high. I remember seeing a Youtube video where the guy stressed tilting the pelvis back. When I try to do this in the water, what I get is an arched back but I don't feel like my body is riding high enough. Should I be striving for this? Is there a perfect balance between pressing down with the chest and arching the back that I am missing? Does anyone have some ideas on how I can work toward this? My balance overall is reasonable but I definitely don't ride high. Incidentally, watching Thorpe film he not only keeps the butt out of water but he swims with a slight tilt (which really confuses me as to how he does this).