If a button item's ButtonStyle property is set to BarButtonStyle.Check, it can have two states (pressed and elevated). The item's state is determined by the Boolean Down property. The Toggle method inverts the value of the Down property and thus toggles the button item's state.
A Toggle method call has no effect if the ButtonStyle property isn't set to BarButtonStyle.Check or if the button item is disabled (the BarItem.Enabled property is set to false).