How do close-circuit jacks work?

These jacks are used in audio systems to patch in a piece of outboard gear. When you plug in the cable, the circuit is interrupted and sent to the external gear. When you remove the cable, the circuit 'closes' and sends the signal on through. A headphone jack is a closed-circuit jack. When you plug in your headphones, the circuit to the internal speaker 'opens' and sound is sent only to the headphones. When you unplug them, the circuit to the speaker 'closes' and you hear sound through the speaker again.