[freightshaker]

BTW, you can reverse 87 & 30 as well as reverse 85 & 86. Think of 87 & 30 as a draw bridge, when the circuit is completed at terminals 85 & 86 the "bridge" is down and the power can cross from the battery (source) to the lights (load).
As for 85 & 85 it's a circuit just like any other. Even a simple light bulb, if you have power going to it will light when the other contact is grounded or vise-versa. It needs power & a ground.
In this case of the lights, the ground is already established and when power is applied from the trigger (blue/yellow wire, running lights, low beam wire etc) this completes the circuit which then lowers the "bridge" and power flows to the load.
In the case of an aftermarket air horn relay, then the power is established either when the key is turned on or on constantly and the circuit is completed when the horn button is pushed, creating a ground and thus completing the circuit & allowing the power to flow to the aftermarket horns.