Just browsing here, before I possibly pick up an Elk. Wondering though, how there's any significance in the last device hooked up in a parallel circuit, wouldn't they all be seen identically from an electrical standpoint? Maybe they need to be wired in a different way? Isn't having each device w/ a home run connected to the same two terminals technically a parallel circuit?
From an electrical standpoint, here's how to picture it:
Imagine you have a zone with nothing but an EOL resistor. This zone is always secure; the panel sees the resistance of the EOL resistor all the time. Now add the power supervision relay, which is electrically in series with the EOL resistor. The relay is closed as long as power is applied, so the panel still sees the EOL resistor. If power is removed from the supervision relay, the relay opens and now the panel sees an electrically open zone (infinite resistance). The panel treats this as a trouble condition.
Now, add sensors to the zone. Each sensor is wired across the two legs of the zone. Since the sensors are normally open, this makes no difference to the zone as long as there is no fire; the sensors may as well not be there. However, if a sensor detects, it closes its internal relay, which shorts out the zone. The panel now sees zero resistance on the zone. This is the alarm condition.
Physically, you want to wire the sensors in a point-to-point configuration, as opposed to a home-run or star configuration. And you want the ends of the wires for each leg to be directly on the terminal screws of the sensor, and not twisted together or anything like that. The idea is that if someone tries to remove the sensor, when they unscrew a terminal screw, the wires will fall out and the continuity down the line will be lost. You want the continuity to depend on both of the wires touching the terminal screw. That's why you don't want to twist the wires together, or pigtail the wires.
Consider what happens if a terminal screw is unscrewed at a sensor, or if the cable is damaged. If a zone conductor is broken, the zone goes open and the panel indicates trouble. If a power conductor is broken, the supervision relay at the end of the run loses power and it opens its relay, which opens the zone and the panel indicates trouble. The panel will only indicate alarm if the zone is shorted, which presumably can only happen if a sensor goes into alarm and closes its relay.