The five second rule was a mechanic which affected mana regeneration prior to patch 4.0.1. After that patch, Addons like this were no longer necessary.
NabuFiveSec monitors your mana regeneration. It displays a bar when your mana regeneration is disabled. This bar display the amount of time until you get your next full tick of mana. It is meant as an aid for casters to use their mana better.
The Five Second Bar is movable, sizable and lockable.
/nfs - Gives you a list of available options.
Thanks for some code samples to: