"Dead or Alive 2 (DOA2) is a fighting game in the Dead or Alive series. It debuted in arcades in October 1999 and was released on February 29, 2000 for the Dreamcast. The graphics and gameplay were enhanced and based on a better game engine, which allowed the characters and stages to appear less angular and more detailed. The story involved a narrative continuation of the first game.
A popular and commonly discussed feature, one credited to Itagaki, was the level of graphical detail Tecmo put into the animated breasts of the female characters. Tecmo went so far as to create a physics engine dedicated entirely to the animation of the female characters' breasts. This feature boosted the overall popularity of the game with its primarily male audience, especially in Japan."
Source: Wikipedia, "Dead_or_Alive_2," available under the CC-BY-SA License.