Relay

You can use the onboard relay switch to control on/off devices that consumer more current than can be supplied by simply turning a digital pin on and off.

The way the relay works is that it switches between connecting a common center pin to one of the two outer pins (State 1 or State 2). So if, for example, you wanted to trigger a solonoid, you'd connect the solonoid's V+ pin to the center pin, and connect State 1 to APM's V+ and State 2 to Ground. In State 1, the solonoid would go on; in State 2 it would go off.

http://ardupilot-mega.googlecode.com/svn/ArduPilotMegaImages/relay.jpg

You script it into missions with any GCS. Here are the commands in the Mission Planner:

http://ardupilot-mega.googlecode.com/svn/ArduPilotMegaImages/relayset.png

Here's some code that demonstrates it switching from one position to another.

{
	pinMode(RELAY_PIN,OUTPUT);
	
	while(1){
		delay(3000);
	
		Serial.println("Relay Position A");
		PORTL |= B00000100;
		delay(3000);
	
		Serial.println("Relay Position B");
		PORTL ^= B00000100;		
	}
}