fire_event_no_wait is helpful for handling modal dialog, that opens by JS scripts via event.
Here is code for that functionality:

watir.rb:
class Element
...
    def fire_event(event)
      fire_event!(event)
      @container.wait
    end

def fire_event!(event)
      assert_enabled
      
highlight(:set)
      ole_object.fireEvent(event)
      highlight(:clear)
    end

def fire_event_no_wait(event)
      assert_enabled
      
highlight(:set)
      object = "#{self.class}.new(self, :unique_number, #{self.unique_number})"
      @page_container.eval_in_spawned_process(object + ".fire_event!(\"#{event}\")")
      highlight(:clear)
    end
...
end