The Duo Sound switch is the name Ibanez applied to a type of pickup modification.
The Duo Sound switch gives the player the ability to split the coils of a humbucking pickup to access a tone more similar to that of a single-coil. It was implemented using a push-push potentiometer. On guitars the switch enabled the inner coil while on bass guitars the coil closer to the neck was activated.
In dual humbucker guitars two Duo Sonic switches were typically employed to allow each pickup to be coil split independently.