Arcane Overflow shows the current overflow of mana based on current procs.
It is based on real current data of procs of the common bufs Power Torrent, Volcanic Potion, Lightweave Embroidery, Darkmoon Card: Volcano, Synapse Springs and also takes into account Blessing of Kings and Mark of the Wild.
The time remaining of the shortest duration is also shown.
It is only shown on Arcane Mages.
Visual configuration is done via the interface menu, also accessible with /ao.