This switch is used where we only need to break a single line in a 120V single-phase supply, providing that it is connected to an ungrounded circuit conductor. There are two wire connection terminals, one for the line voltage, and the other for the “switch leg” to the lamp. It does not matter which wire is connected to which terminal as the switch acts as a simple “open/closed” device.
When the external operating handle is operated the internal switching mechanism either opens or closes a single line contact and thus controls the flow of current through the device and any other series-connected loads.
The switch has a marked “on” and “off” position and switches should be installed so that moving the handle upwards energizes the load, and moving the handle down de-energizes the load. As a general safety rule, gravity should never be able to accidentally energize a circuit.