These triggers let you perform basic tasks such as springing an ambush as the player walks down a corridor, or waking up and orienting monsters to the approach of the player. They have a number of fields in common with doors and buttons, so what you already know about doors will be helpful, and the buttons reference page should be easy reading when you've worked through this.
To begin with, both the trigger_once and the trigger_multiple own a brush (or brushes), and they fire (trigger their targets) when a player or monster touches one of these brushes. The difference between them is the that trigger_once removes itself from the map after firing, whereas the trigger_multiple stays behind to fire again. They use the same code, so have the same fields and spawnflags. The one difference is that a trigger_multiple should have a wait field telling it how long to wait before firing again if the entity touching it and causing it to fire is still in contact (by default, it re-fires after .2 seconds, which wastes cycles and can produce bizarre-looking effects).
Some simple fields these triggers have are message, whose value is the message number of a message to be displayed when the trigger fires, soundtype (three numerical values, see the reference page), and the puzzle-fields puzzle_piece1...3, and no_puzzle_msg, which work the same way as they do with doors. And, of course, killtarget (which seems to work with any entity that can trigger another).
A more notable one is delay, whose value is the interval (secs) between when the trigger is triggered, and when it actually fires (this is probably not so useful for these triggers, but is very useful for trigger_relay, which we'll be getting to shortly).
There are also two simple spawnflags, 2 - MONSTERTOUCH, which is supposed to cause the trigger to be fired only by monsters, and 16 - REMOVEPP, which works just like with doors.
So for excercises you can try to produce a monster that is woken up by a trigger_once, or a door that is open when the player walks through a certain part of a corridor.
Two additional new fields are siege_team, with values 1 for
defender and 2 for attacker, and impulse, with value 33.
A siege_team-value makes the trigger operable only by a member
of the specified team. This could be used in to make a door that
could be opened only by members of one team or the other (but maybe
bashed down by members of either team). In impulse 33 field
makes a trigger that doesn't fire until it `used' by the player
(the
In addition to these trigger_relays occur in the maps with
a number of fields whose function is unclear, as detailed on the
reference page (these might be leftovers from earlier versions of the
entity system).
Relays
trigger_relays add a lot of flexibility to triggering, and can
be used to `script' complex events (but a real scripting language is
better, say the Raven designers who are using the one that Rick Johnson
has developed for Heretic 2 and Soldier of Fortune). The idea of a
trigger_relay is that it fires its targets when it is itself
triggered. So the fields that a
trigger_relay needs
are target and target. A delay field can be
used to arrange sequences of events separated by suitably portentious
intervals. For example one event might trigger a sound
(sound_maker), and also a trigger_relay
with a delay-value of 3, which would target a door.
So first the sound will be generated, then, three seconds later, the
door will open.