Programmed/animated in Flash from scratch. The particles fill defined rects (body, arms, etc.) dynamically. This is most noticeable when first loading a character, since the particles are initialized randomly and correct their positions over time.
The characters themselves (the invisible body boundaries being filled with the particles) were animated with Flash motion tweens and whatnot by hand. May be very taxing with some of the more complex scenes.
Link to the short animation "Long Distance" below. Created over the span of about a week, along with modifications to the particle system that were needed for it to perform reasonably.