I'm a fairly casual amateur swimmer (who happens to have a PhD in physics, which is surprisingly helpful if you want to reduce your drag in the pool, but I digr

I'm a fairly casual amateur swimmer (who happens to have a PhD in physics, which is surprisingly helpful if you want to reduce your drag in the pool, but I digress). I learned this technique in 1998 or 1999 from a Yale or Harvard swimmer home on vacation in Sierra Madre, CA. He was doing this crazy sideways kick thing, and when I asked him about he said his coach had them doing it because if you tried to do a pure kick version of the butterfly from a normal face-down position you lost too much range of motion in the small of your back. I already knew other coaches encouraged on-your-side flutter kick practice to get more comfortable swimming on your side (most of the drag you experience has to do with surface interactions, so rolling your body to get it farther from the surface helps with drag, just like submarines can commonly go faster than surface ships). I've been hooked ever since. It's surprisingly fast, even if you're not in great shape, which is lots of fun, but it does take a little practice to get the hang of breathing while doing it. If you want to give it a try, my advice is to start with a kickboard for balance in the beginning, then toss the board once you get the hang of the position because you don't really need it and it just slows you down with extra drag.