On a successful spellcast, a mage can use their will to limit excessive use of Magicka. This is known as Spell Restraint or restrained spellcasting. A character can choose to reduce the cost of a successfully cast spell by their Willpower bonus to a minimum of 1.
While typically this is desirable, sometimes not holding back with a spell can have extra benefits. Some spells have the Overload attribute, which will state certain extra effects the character can benefit from if they choose not to restrain the spell when casting it.