The director in L4D was had a very simple logic with inputs like number of alive special infecteds and number of zombies to control spawns. It's like seeing a health kit when your health is low.
IIRC it tried to be slightly more clever (might've been L4D2) in that it also kept track of the recent history of the game and attempted to balance the pace of the action.
If it thought players had had it easy for a while it might throw a larger waves of monsters and specifically deny spawning health for a few minutes.
There's a good presentation somewhere about how it would try to balance the rising action with periods of "stress" and "relief" over the course of a level.