-the holograms are created by computer-controlled and monitoried lasers that are projected from every surface of the room, creating a realistic 3-D picture. those training in the Holo Room wear special gloves and/or boots containing sensors. when the sensors on the hand or feet come into contact with the lasers, the computer records where the sensors "hit" the lasers, then calibrates the reaction of the hologram (the total laser image) to correspond to the part of the hologram that was hit
OR
-the setting in the holoroom is different mechanical parts of the room that rise and fall depending on teh setting. the "people" in the holoroom are robots that have projectors built under unbreakable plastic but still project the image of the "enemy" you're fighting up against. like the idea above, the projectors underneath the plastic also are sensors which record when the image/laser is hit/disconnected, thus telling the robot to react in a certain way
and yes, I did just sit here and think of that on my own :)