Hi Nicky,
Butterfly ?
Do you mean Dolphin Kick ? - (Kicking with both legs together)
No Idea how you would do a butterfly stroke with you arms underwater. Think the kick is similar though.
I use both Dolphin kick and flutter in the Ocean and in training. Dolphin kick works great if your legs are worn out, you use your back & stomach muscles more. It's good to know a few alternate kicks, in case of fatique, cramps etc.
Also once you get the hang of the Dolphin kick it feels great, works best with long blade fins, specially carbon fibre ones.
This is about the time that all the Monophites will jump in and tell you how good Dolphin kicking with a Monofin is. It is a lot of fun, I've tried it, get so much speed with them. It makes my Picasso Carbon fibres fins slow by comparisson.
It's easy to get much more speed from a Dolphin kick then flutter. It takes a bit of practise to get a slow efficient kick with it.
Try doing both in training, I do one lap with Dolphin kick, then come back with Flutter....
In the Ocean, try a Dolphin kick on the bottom, or coming up from a dive. Work really well.
Getting down with a Dolphin kick is the tricky part.
Cheers,
Wal