“Expeliarmus” is one of the spells students learn at Hogwarts early on. The object is simply to disarm your opponent, to make his wand fly out of his hand so he can’t use it. So it is taught early because it is harmless and useful.
As school advances, other things are taught–more aggressive and dangerous spells. This is part of the mystery. Part of the enchantment of the books is that they suggest a plausible advance–without ever explaining how it works. None of us come out of the series knowing how to do magic, but that’s hardly the point: the point is that the characters’ advance is plausible. They learn more difficult things. And hints are dropped about the nature of that magic that make it more satisfying. There is a gathering sense as you go along that the character of the person influences the result of the spell cast. It can be an obstacle or it can be an aid as with any skill.
With the return of Voldemort, two contending notions begin to be shaped. Voldemort is a kind of virtuoso magician who excels in what he does because he is cruel, unscrupulous and of uninhibited ambition. His idea of magic is pragmatic: he is no virtuous magician, but he is a great master of means. Against this is set the idea of one who is better than he is, but not by virtue of his great mastery of lore or magic. Harry is a person supremely unconcerned with achieving academic excellence or carefully planning the successful stages of an anticipated career. He is caught up in events where faced with moral choices he often decides to his personal disadvantage, though never to the disadvantage of his character: he is decent, he is loyal, he is brave, he is tormented by having brought harm to his friends by his choices. He is a master not of means, but always has in view the greater ends. This grows. The plot overarching seven volumes is an exposure of the character of Harry Potter in action.
And it is with the weight of all we have learned about him that we read his spell at that crucial moment in that final duel. How like him when faced with life and death to remain innocent of the importance of means! I remember the weak joy anticipating defeat of that moment, the sense of “Oh Harry!” before the onset of disappointment is dispelled. He is blinded by ends, and so achieves–with all the force of everything the seven volumes have been leading up to–the triumph that alone satisfies the persevering reader.
Voldemort is a virtuoso magician, but in the end, as in all good literature, character triumphs. Means are the expression of an intent, and it is the greater intent not the more glamorous means that will prevail. Harry Potter is the virtuous magician, and for that reason, even though it has not been the supreme concern of his existence–as it has of Voldemort’s–to prove that he is the greater magician, for that reason we see clearly that he is. So the lesser means: “Expeliarmus!” proves the means to the greater end.