Hi Tom, I'm thinking it's Particular. One of the smokey/watery effects from
http://people.redgiantsoftware.com/ should be a good start. To get a slightly more hard-edged result as per the video, you can reduce the feathering on the built-in particle type. Another method to achieving a hard-edged result is to apply a wee bit of blur (to spread out the edges) followed by either Matte Choker or Simple Choker to choke and harden the edges.
HTH
RoRK
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