An aux switch, or auxiliary switch, is a secondary switch on some custom lightsabers that activates an audio or visual effect. By comparison, the activation switch, or main switch on the lightsaber, turns on and turns off the lightsaber.
The aux switch is commonly located below the activation switch on a lightsaber, but may be located to the side or elsewhere on the hilt as well. Some lightsabers use a strategically-hidden, discreet tactile switch for an aux switch.
The function of an aux switch may vary from lightsaber to lightsaber, depending on the capability and configuration of the lightsaber controller/sound module. An aux switch will commonly activate supplemental lightsaber features like: blaster bolt deflection, lockup sound, Force sounds, etc. An aux switch may also change the blade color (custom saber company Kyberlight [affiliate link] uses an aux switch for this purpose).
COVER IMAGE Kyberlight [affiliate link] lightsaber hilt with arrow pointing to the aux switch