The discovery is that the players exist in your game in two instances, the "avatar" and the "organism". The avatar is, of course, what is rendered on screen; the organism is what is left of the player after passing through the plane into the game world -- the preservation the game's functionality and its translation into in-game abilities.
In nature, there is no "avatar", since a creature's body is an exact representation of the organism's capabilities. In games, however, there is an inherent discrepancy between the two -- since the developer dictates in detail the embodiment of the organism. The game might dictate that the hero is a space marine; the challenge, however, is to design a user interface -- or let's call it "organism interface" -- that makes the player feel adequately that he or she is a space marine.
via gamasutra.com