RedZone gives you the ability to detect players or mobs in user-placed zones.
Basic Idea
A RedZone is sort of like a giant pressure plate. When a player enters the region (even if flying) a signal is sent to all the RedTriggers registered in that zone. When signaled they will either power on or off depending on their settings (You can also make toggles and nor-latches).
A RedTrigger is a way to easily power items inside (or outside) a RedZone. It can also be used as an input to trigger a zone remotely. When a regular output trigger is signaled it responds by changing the state of an attached lever, allowing you to draw power from it (you can also use torches as they will be swapped with redstone torches on signalling). If you have used CraftBook, you already know how these guys sort-of work.