It will do the following to inform you that an ability is ready:
Flash the edges of the screen. You can disable flashing globally or per ability.
Display the ability icon with a OmniCC-like counter on it, showing you which ability you can use, and how much time you got before the proc fades and you can't use it anymore.
This addon is perfect for rotation altering. You have a set of spells in your rotation. You get a icon with a timer when you can use an ability that isn't in your rotation. You can now decide when and how, if at all, you want to alter rotation from the information this addon delivers.
As it's in beta, there are some inconsistencies. For instance, when sudden death procs for a warrior, the sudden death icon will appear, instead of the icon of the ability that's actually available -- execute.
Here's a short video of the addon in action: http://www.wegame.com/watch/Example_of_how_the_Nao_addon_works_2/