It's Sanji history time (with pictures!).
While reading through (sooo many) pages of the closed 415 topic I noticed a lot of people whining about "explanations" on the demon wind leg technique.
So I decided to sit and think instead of posting, regardless of whether I would defend of stand with these posters. I mean from a subjective point of view everything must have an explanation.
So here is mine:
At first we saw Sanji's simple spin kicks, it's a trademark Zeff, and it simply allows him to hurt any surrounding opponents while also increasing the power of his kicks (due to accelarated velocity of the spin).
http://i69.photobucket.com/albums/i64/refii/takis2.jpg
On the sea train we see Sanji use the same technique, except instead of releasing energy uncontrollably in all directions, he focused the accelarated force of his kicks into a single point, namely Bluenos side.
http://i69.photobucket.com/albums/i64/refii/takis3.jpg
Finally, Sanji took the evolution of these techniques a step further, except this time he doesn't release the energy at all. Instead he holds the static energy into his legs until it is time to release it with his kicks, much like a battery holds energy.
http://i69.photobucket.com/albums/i64/refii/takis7.jpg
Now while can't explain how a human can store energy like that to be released, I also can't explain how swordsmen/CP9'ers can cut the wind, so I think it's fairly self explanatory One Piece logic. Also the flame design is just 2 cool 4 u to handle if you won't accept Zeff's divine intervention burns it onto Sanjis pants :p
Just something to think about, and while we're on the subject of thinking:
http://i69.photobucket.com/albums/i64/refii/takis4-1.jpg
Despite his vow as a Chef, it's in his blood since he was a child, I doubt this is the last time we've seen Ero-cook san use this technique.