Function or “Fn” mode is the default mode for the DT402 & DT400 throttles because it is the mode that runs your trains.
In Fn mode, the throttle knobs, reverse direction keys and Y+/N- keys are used to control locos. The numeric keypad is used to access functions for the currently active loco address. In Sw Mode, the keypad is used to operate track switches. This means that you can continue to control your trains while turning on and off functions and operating turnouts.
If a throttle has a locomotive address selected, the associated loco icon and direction indicator will be lit. In addition, if the smoke above the locomotive icon is flashing , this indicates that this is the currently active throttle meaning that the Function keys on the throttle and the Function Indicators in the display are associated with that address.
To make the other throttle active, simply turn or click the other throttle knob.
The smoke icon will start blinking on the other throttle and the function information will be displayed for that loco address.