Mains-powered instruments such as keyboards are grounded via the mains connection. Since the DI box is also grounded, via the mixing console to which it is connected, this can result in the dreaded hum caused by ground loops. The ground lift switch solves this problem by isolating the ground connections of the two devices. In practice it is not necessary to be concerned about the complexities involved; if there is a hum, simply use the ground lift switch.