A single pole, double throw relay (SPDT) is a standard relay. It has 5 terminals on the bottom. Each terminal has a # by it. 85 and 86 are the control terminals, it doesn't matter which is wired to power or ground. When power goes to this circuit, it activates a magnet inside. 30 is the output terminal. 87 is the input when at rest (not activated) 87a is the input when activated. What ever is hooked to 87 or 87a will come out of 30. Think of it as the giant hand thrown switch used by hollywood to throw power to the electric chair, only activated by a magnet. If the relay is used to turn something off (interrupt) then power is hooked to 87, 30 goes to the accessory. When the relay is activated (12v to 86, ground to 85, pulls magnetic switch) the sweep will go from 87 to 87a (which has nothing connected to it), thereby breaking the circuit. If used to power something, (ex. driving lights) it will keep high current from going through the switch and melting it down. Wire 12v from switch to 86, ground to 85. 12v from battery (fused) to 87a, nothing to 87, 30 goes to the lights. Low current flows through the switch, activating the relay, sending high current to the lights. If used to power a trouble light, such as when the oil pressure switch kills the engine, wire 12v from the pressure switch to 86, ground to 85. 12v to 87, nothing to 87a, 30 sends 12v to the indicator light. When the pressure switch trips, 12v is cut from the sweep, which sends it from 87a (nothing) to 87 (12v), sending power to the lamp. Now you won't drive yourself crazy trying to troubleshoot the ignition and fuel, when you suddenly remember to check the oil. Once you wrap your mind around the concept, relays can be used for many things. Remember, they are your friends!