Currently, the "Tab" key switches the bar and this action is configurable.
Shift + the configured key switches the bar in reverse order.
By default, Shift+Tab is the Steam shortcut to open its ingame overlay (and send invites to friends, for example).

If one wants to avoid this conflict and keep the default Steam shortcut, this would be nice if we could have a new configurable entry dedicated to the "reverse switch bar" in the key bindings.