Are you talking about the 'dancing raindrops' or
those 'red bloody dancers'? :)
Either way, the main ingredient is the Shade/Shape plugin.
The basic idea is, you create some mattes
and animate isolated objects together and bingo!
(Exception: the big 'splash' at the beginning of
the red dancers used a stock footage instead of
keyed dancers, though still similarly processed
using S/S. I was like, WTF?!, when I first saw it,
until Pete explained it to me. LOL)
Anyways, it's pretty easy to create illusions of
anything from metallic or liquidy reflections
to something like artificial shadows for very flat
(ex. cartoon) objects.