So I'm a "late onset" 55-year-old swimmer who has been edging toward competency in fly for a couple years. I've read and been told to keep my butt up so as not to drop my hips. I've found it hard to do that without kicking too hard (to lift my butt) and losing the fairly flat stroke needed to keep forward momentum. But an offhand comment by a fellow swimmer helped me out and perhaps will help others. He, an experienced swimmer, was about to do a fly event at a meet, and mentioned to me that he had a slight knee injury. He said he was going to keep his core tight while doing fly so as not to overdo the undulation and kick, in order to keep from injuring his knee. I put that in practice over the next few days and found that it helped to keep an undulation going but also to flatten out my stroke and keep my rear end near the surface. So if you're trying to help someone learn fly, and they're dropping their hips, you might suggest they tighten their core a bit and see if that helps.
