Here's a question: how do swim fins work? I mean, it seems obvious, but when I start to think about the forces at play, and draw vectors, etc, things actually

Here's a question: how do swim fins work? I mean, it seems obvious, but when I start to think about the forces at play, and draw vectors, etc, things actually get less clear. Anyone have an intuitive but rigorous answer? (My question reminds me of the "how do we suck in spaghetti" question, which once again seems kinda dumb but then is frustrating when you start to break it down. http://www.straightdope.com/columns/read/1260/how-does-one-suck-in-a-piece-of-spaghetti http://www.straightdope.com/columns/read/1260/how-does-one-s... )