It's a really cool and sneaky effect. I think they load a fading JPG, then subtly swap it out with the video once it loads - which is exactly the same image / size. The JPG is probably frame 1 of the movie, so the smoke doesn't jitter.
Swampy Freddie is from here :) http://mailchimp.com/replyall/
I like they're keeping the theme, though I still think the last one was absolutely perfect.
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