The Mode button will cycle through the four settings for the Flush Fit Register Booster. When the mode button is pressed and the "ON" light is lit the automatic thermostat function is bypassed and the fans will remain on. If you want to circulate a small amount of air this is the mode to use. Your Flush Fit Register Booster is equipped with a highly sensitive electronic thermostat. When the Mode is in the "HEAT" or "COOL" setting the thermostat will control the on/off of the fans. In the "HEAT" Mode the thermostat will automatically turn the fans on when it senses heated air after the furnace starts. Similarly, when in the "COOL" Mode it will turn the fans on when it senses cooled air after  your air conditioner starts. This operation is fully automatic and you have a control on the function panel to fine tune when you wish the Flush Fit Register Booster to start and stop.