They cannot divert water to the emergency, or “auxiliary,” spillway. It is simply an overflow point and goes into action when the reservoir level rises to it. Like your bathtub would overflow if the water reached the top.
It’s confusing because much of the media is referring to the “main” concrete flume spillway as the “emergency” spillway.
But as I understand it, there is another earthen spillway running roughly parallel to the main concrete one. I guess that one is higher than the main concrete one. I would assume that one would be the emergency or auxiliary one, but the media makes it really confusing.