With passive interaction all interaction is meant that is not performed by the user via his input device. This interaction is mainly the movement of the user inside the CAVE which is tracked by the motion tracking system.
This interaction is done nearly all the time, everytime the user walks around or just turns his head. It is important to note the relevance of ths interaction as only by correct motion tracking and its correct evaluation an exact projection of the VR can be calculated.
An improvement to motion tracking could be accomplished by an eye tracking system. If the application could not only track the position of the user but also exatly where he is looking, even more realistic frames could be rendered.