If true, the menu item assumes the selected state regardless of the current WAM/Webroutine or the is_selected_if_also property. If you want to apply your own logic to deciding the state of the menu item you can set is_selected_if_also to False to disable the default logic and place an appropriate Boolean expression into force_selected.
For example, you could create a field that contained the name of the selected menu item. Then you could set the force_selected property (using the XPath entry area) to
#SELMENU = 'Menu1'
False() (the default logic is applied)
Any valid XPath expression that returns a Boolean value.