I have used the Swimmer's Snorkel, by Finis, for many years as a breath control and stroke technique device. I have used it mostly in Freestyle, a little in Back and Breast, but very little during Fly. Last week I observed on of my college swimmers using one for an entire set of fly. I had to ask her about it afterward to see what her thoughts were. She said it allowed her to swim more fly through a set without breaking down. I however, noticed something more, but thought I would give it a more thorough try with a larger group...my senior boys.
Last Friday I gave a set of 25's Fly with snorkels and fins. Throughout the set I encouraged the boys to keep their stroke profile, there stroke undulation, as tight as possible. I really wanted to encourage them to engage their core more in the process of each stroke cycle. When you have the snorkel on, you don't have to recover up to get a breath, but it also encourages a shallower downward motion as the hands are driving forward. Undulate too deep, and your snorkel fills. Recover to high, and you may as well not have on a snorkel because your face will be out of the water.
It was a real challenge for some of the boys to tighten up that body undulation. However, once they got the hang of it, they began to use there core muscles (abs, lower back, upper legs) more in during each stroke cycle. They were using there heads and arms less, and using their core more. The result was more forward velocity in each stroke and fewer strokes per length.
I am sold on using the snorkel for fly, and encourage you to give it a try.