One of the more interesting questions I get from Muggle readers has to do with the flinching that seemed to always accompany the name "Voldemort." Some have suggested that Ms. Rowling was simply playing up the psychological fear. But she was not.
This is a little difficult to explain, but it might help if you begin by thinking about words and spells and how they work.
The "accepted" magical spells taught at Hogwarts and most other schools are derived from Latin simply because they were formulated and regularized at a time when Latin was a nearly universal language in Europe.
But the most original Witches and Wizards I have known, such as Dumbledore, Flitwick, and McGonagall, preferred other languages. Dumbledore rarely spoke incantations, but I have strong reason to believe that when he did, he generally used Anglo-Saxon, probably West Saxon to be more precise. Flitwick (whose parentage I never did discover) could cast charms in a score of languages but favored Welsh for some reason, maybe because Flitwick had a passion for Merlin. McGonagall, predictably, only used the Latinate spells when she taught and used Gaelic at almost all other times. We Gryffindors frequently heard her conjure light or lock doors in Gaelic. To be called to McGonagall's office when she was irritated generally meant learning some new Highland incantation, and while she could transform into a cat without words, I occasionally heard her mutter in Gaelic as she made the change.
In short, as with wands, so with language: the Witch and her language need to be comfortable together, and to grow together.
Usually, one discovers one's magic without any particular spell, simply by influencing the world around one in stressful or joyful moments. "Emotion is magic" at that age, says the magical child-development specialist Eliza Halfwand, and she is quite right.
But just as Muggle children learn how to associate words with desires and other feelings, so young Wizards and Witches learn to channel their magic, through words and eventually wands. Harry vanished glass when he was eleven before he knew the "proper" incantation, and at Hogwarts he learned to summon, control, and channel that emotion, in part, by focusing the magic through a learned word: "Evanesco."
Had he not learned the word, he probably would have found one of his own. All magical children make up their own incantations. Tom Riddle was exceptionally creative and disciplined at it, and in his Muggle orphanage had developed quite a range of verbal spells by the age of eleven. Probably many of his spells were in a mixture of English and Parseltongue, though he was always secretive about this. His childhood spell, "Tell the truth!" was effective enough for his purposes, but inelegant and clumsy and put his victims on their guard. (Typically the shorter the spell, the better it channels simple magic.) Dumbledore once told Harry that discovering the word "Crucio" was a revelation to Riddle: whatever word he had employed up to that time to cause pain, he found in the older "accepted" spell a word that much more aptly channeled the cruelty that came so easily to him.
What's extraordinary is that with use, some words take on weak magical properties of their own. "Crucio" is a powerful word in the Wizarding world, and even magical people who do not know the curse (there are many) become a little uneasy on hearing the word. Snape's "invention" of the "Sectumsempra" curse was remarkable in that he, like Voldemort, was very talented at imprinting the power upon the word itself. Had Harry understood what he was doing when he cast the curse at Draco Malfoy, he would have probably killed him, but Snape (typically for him) was enormously careful that Magic itself should remember the word, and he let much of his magical power influence his choice of the word: in other words, he cooperated with Magic. Even without knowing the specific function of the curse, Harry was able to cast it in a crude form simply by channeling his emotions through the word: the curse, as it were, remembered Snape's intention, and it nearly killed Draco.
Such a word has something of the effect that onomatopoeia has on Muggles: when the word has been invested with magical significance for generations, it simply comes to "sound like" the emotion that it helps to channel, all on its own, without any cognitive awareness by the Wizard or Witch.
Consequently, it has been reported that many young magical people who read the Harry Potter books before discovering their own magical nature began to suspect they were magical because the spells felt oddly familiar and evocative. I got a letter from one Muggle-born girl from Kent, who is now at Hogwarts, who had such a strong reaction to some of the more basic incantations that she actually performed them while reading the books.
Magical theoreticians argue quite a lot about why this is the case. Some say that the words precede the spell, and that you could no more conjure something with the wrong incantation than you could make a cake with the wrong ingredients. These incantationalists believe that words feel appropriate to the magic because they simply are the right words. In their view, Snape did not invent Sectumsempra, he merely discovered it.
Others argue that magic must proceed from the Wizard and that the incantation focuses his power just as (for instance) music focuses feeling. In that view (which is closer to my own), another incantation, or no incantation at all, could do the job, if the wizard were strong enough of will. In this view, it's a bit of an art to find the word that best channels one's intentions.
But this theory suggests that "magic itself remembers" the word: and this is a very tantalizing thought, as it suggests that the mysterious force of magic has a personality, that it cooperates with Wizards and Witches. This accords with my own experiences.
This also explains something of the power of Voldemort's name.
Witches and Wizards have extraordinary sensitivity to that name because it was specifically chosen for its magical properties, which were actually enhanced by the evil magic that Tom Riddle invested in it.
When Riddle came to power, he exerted a tremendous amount of power to imprint a magical fear upon his own name, investing it with terrible qualities that magical people feel. In my view, Voldemort imprinted dark fear on Magic itself, channeling fearful spells through the incantation of his own name, until Magic just remembered that fear. It was by pushing that memory to extraordinary, fearsome limits in his last year that Voldemort made his own name "taboo," turning a name into a word that could undo ordinary magical defenses.
Once Voldemort had done that, Ron sensed in the name a horror and fear so intense that he knew it could crush the feeble spells we had put up for our own protection. It's quite likely that Voldemort committed murder or human sacrifice to enable the taboo, and only self-sacrifice, such as Lily Potter's, could have stood up to so powerful an invocation of dark magic.
I felt the horror, too, but I confess that I was trying to live up to Dumbledore's memory at the time; I had, so to speak, talked myself out of the fear. I dimly grasped the power of love that Dumbledore understood so well, but it was probably foolish of me to pretend that the horror of the name was inconsequential: Ron was quite right when he said that we needed to show Voldemort "some respect" in this regard: we were not Dumbledore, nor did we have Lily's sacrificial charm protecting us.
There are a few theories about why some magical people could hear the name without flinching: in Dumbledore's case, I think that he was himself so steeped in magic, and so familiar with it, that he was impervious to such things as magical fear. He understood that "perfect love casts out fear," and more than any Wizard in history understood the precise power love has to undo spells of fear and hate and cruelty. In other words, I believe he had so much magical power that contradicted fear that his own "imprint" upon Magic more than cancelled out Voldemort's influences on the word.
I think that Lupin (who was quite diligent at practicing defensive magic) mastered that same magical dynamic, probably through pure application. By the time we met him, he used The Name with an unaffected casualness that could only have come by mastering the magical cause of the fear. When he praised Harry for "fearing fear itself," he doubtless had in mind not simply ordinary fear (which is bad enough), but those fears that affected and even poisoned magic.
Sirius, on the other hand, was simply courageous, defiant, and reckless, and probably felt the power of the name as fully as anybody. But he (and probably James Potter) truly enjoyed facing fears, and would have found it dull and contemptible to surrender to any fear. They seem to have spoken the name with the same exhilaration Muggles feel when they jump off skyscrapers with parachutes.
As for Harry, of course he had a fragment of Voldemort's soul in him, which probably insulated him from that dread somewhat. But I suspect that his mother's charm protected him from the fear of Voldemort's name, just as it protected him from Voldemort's hatred: what Dumbledore understood, Harry experienced. In both cases, it was inevitably love that protected them from fear, and enabled in the end Harry's self-sacrifice.