Mindduels are magical, psionic battles between two mages, initiated with the aid of a focusing mechanism such as a smoky crystal. Over the course of such a battle, magical energy flows back and forth, until eventually one mage comes away with a significant chunk of the other's energy. The losing mage may end up dumbfounded or, in extreme cases, dead.
No spell known can shield a mage from a mindduel; thus, they are a very effective way to disable enemy magi. However, minddueling is a complex art, not easily mastered. Furthermore, it bears little in common with the traditional magical arts, so a magically apt archmage may not be a particuarly good mindduelist.
Rings of will, which were created to protect against dumbfounding, have the "side effect" of making their wearers much better in a mindduel.
Solberg was a master mindduelist.