Have you guys tried the "director_show_intensity 1" command? It's pretty neat, it gives you intensity gauges for each of the players. This basically gives you an inside look into what the Director is "thinking". You can see these gauges off to the right, next to the inventory slots.
^ "BUILD_UP". As the name suggests, this is when the Director decides to start increasing the tension. It has started off with a Smoker at the other end of the room, and some music.
^ "BUILD_UP". All these zombies push the intensity for the players higher.
^ "BUILD_UP". Boomer vomit and the subsequent zombie rush can push the intensity up very quickly. I imagine the other bosses are the same way.
^ "SUSTAIN_PEAK". This is more or less the climax of the action. The Director will try to sustain the tension here for the time being, but won't try to keep pushing it even higher. At this point, the players are assumed to be fighting for their lives.
^ "PEAK_FADE". This is the peak slowing down. The Director determines this level/length of tension to be sufficient, and so starts phasing out of heavy action.
^ "RELAX". This is the "come down" from the peak of heavy tension. No ambushes, no hordes, no heavy fighting. This is the space of time that the Director gives the players to recover from the previous action. Without these spaces, the game would likely be exhausting. As this relaxation continues, the tension bars get near the bottom, and when they do, the Director will prepare another build up and the cycle shall repeat itself.