Immediately after rolling the dice to cast a spell or to dispel an enemy spell, the Wizard wielding the Staff of Volans can pick up one of the dice rolled and turn it so that its result is one higher than the number rolled.

Only then is the total score and its effects worked out. Including Miscasts and Irresistible Force. This is especially useful because the Staff will make Miscasts and failed Dispels rarer (you need to roll at least three 1s) and will make spells cast with Irresistible Force more common (if you roll a 5 and a 6. you can transform the 5 into a 6).