BOSTON — Backstage at Symphony Hall here on a recent afternoon, Hilary Hahn opened her violin case and took out her instrument.She flipped it up to her chin, then paced around; she was warming up to play Bach for a group of Boston Symphony Orchestra staffers, as a run-through before she set out on a tour that continues in Los Angeles and Chicago this week. For the moment, she was trying to break in a new set of strings, as any violinist might.She paused. She set her pink-cased iPhone down to face her, having scouted the dressing room for an angle, then turned on its camera and pressed record. She played her Vuillaume violin toward the lens, but not exactly for it. She let it watch while she tuned and tuned again; while she repeated tricky little passages; while she sighed, composing herself. She stopped it when she was done.Hahn edited the video down to a bit more than a minute of unflashy content then posted it, with all the brisk efficiency of a social media intern, to Twitter, Instagram and TikTok. There were no retakes, no notes to her publicist. Season 6, Day 61, of #100daysofpractice was in the can.“I make a point of not picking up the part of the practice that is impressive,” Hahn, 43, said in an interview afterward. “I pick out the part that’s the actual work, where I know I was in the zone, and I wasn’t thinking about anything else.”Hahn, the artist-in-residence at the Chicago Symphony, has long thought about her role in broader terms than many superstar soloists. She has commissioned works including garlanded concertos and brief encores; taped Suzuki exercises for young students to aspire to; and given recitals for babies (all right, their parents). And this prodigy turned pre-eminence is an experienced poster, too: For years, she tweeted in the voice of her violin case.Even so, #100daysofpractice has become an unexpected phenomenon. Social media statistics are notoriously unreliable, but the hashtag counts 800,000 posts on Instagram alone, and has brought amateurs and professionals alike into a community of musicians who, for their own reasons and in their own ways, post part of their daily routine. Drawing back the veil on how musicians work when they are not onstage, Hahn is trying to relieve at least some of the negativity that can surround a crucial — yet traditionally private and largely untaught — element of a musical life.Hahn’s practice videos tend to be recorded where they can be — whether on the road, in a hotel room or at home.Sophie Park for The New York Times“I make a point of not picking up the part of the practice that is impressive,” Hahn said.Sophie Park for The New York TimesHahn came up with the idea in 2017, when she first noticed #The100DayProject, an initiative that asked creative, primarily visual artists to make something, day after day. She chose an activity that she thought she should have been undertaking with a similar commitment to regularity, but was not.“I desperately wanted to get reposted, get attention,” Hahn, laughing, recalled of a time when her social-media presence was not as formidable as it is now. “I didn’t get reposted at all, I was like: ‘I’m here! I’m doing something innovative! I’m boring my fans! Notice me!’”On one level, Hahn’s posts since are a diary of a virtuoso’s life. There’s Hahn at the Teatro La Fenice in Venice, at Wigmore Hall in London, at David Geffen Hall in New York, where she recently became the first artist to play a solo recital in the refurbished main theater. There’s Hahn on a private jet, in a hotel, and in another, and another. There’s Hahn in her Cambridge, Mass., home, with her Grammys on a side table or her guinea pigs behind her. There’s Hahn the working mother, playing with one of her two children as her impromptu accompanist, or stealing a quiet moment after midnight, exhausted.Part of Hahn’s message, she said, is that being deliberate about practice, whatever else might be going on in life, allows marginal gains to compound. That opportunity for accountability and self-discipline has attracted other soloists to join in. The pianist Dan Tepfer said that he adopted the hashtag this year to recommit to daily practice, after a grueling, monthslong tour.“I like to say that if your practicing isn’t a practice, you’re not practicing,” Tepfer said. “It truly is a practice, it’s a daily activity, and the power of practicing comes with that kind of continuity.”Hahn practicing on the stage of Symphony Hall in Boston.Sophie Park for The New York TimesHahn initially saw the project along similar lines, and to an extent still does. But as she read the replies to her posts, and spoke with fans after concerts, she saw that the posts were being interpreted as a statement about the need for musicians to accept imperfections and embrace their vulnerabilities — or as a challenge to “the toxic mentality around practice,” as she put it.“We’re just so often in classical music, really trained to beat ourselves up until we get it right, on our own,” Hahn said. “I compare it to walking into a room by yourself, and you’re looking in a mirror, and you’re having to pick out everything that’s wrong with you, and then fix it, with no ability to fix it. You’re supposed to walk out better. And it’s just such an impossible thing. You actually just walk out with all these ideas in your head of what’s wrong with you.”“I realized that we need to have a lot more self-compassion as musicians,” she added. “You can’t become someone you’re not in practice, and you can’t make the music become something that it’s not ready to be. It’s just difficult, though, to reconcile that with expectations, sometimes.”Hahn’s most powerful videos are not those in which she tosses off some Bach with all her familiar assurance, but those in which she does least with her instrument. “Practice” turns out to mean all kinds of things, from listening back to past concerts to doing near-silent left-hand studies while the laundry whirs along. But it can also mean mindfully taking a day off, or acknowledging feeling burned out, and responding appropriately.“I know some people say that’s not practice,” Hahn said on the video for Day 34 of last year’s series. “Can you count that as practice? But it is about the practice of long-term practicing, that mentality that it is — it’s a lifestyle. There’s a consistency to it, and being a consistent practicer doesn’t always mean practicing by data.”Hahn’s videos, she said, challenge “the toxic mentality around practice.”Sophie Park for The New York TimesMany of Hahn’s admirers have taken that lesson about mindfulness to heart. Another violinist, Elena Urioste, tried the project two years ago, and “promptly failed on my third day,” she wrote on Instagram. She responded with her own hashtag, #ErraticDaysofPractice.The rising violinist and hashtag devotee Nancy Zhou said that Hahn “positively reinforces the whole practice culture and what it should be,” and that she was “completely confident” that the star has had an influence.“It makes them start thinking,” Zhou said of colleagues she had talked with, “about, well, how can they more deeply and more forgivingly look at the way they practice?”Hahn said that series has been useful to her own routine, though it took her time “to be at one with the public and the private aspects of it.” And there have been periods when filming — or writing analytically about it — has interfered with practice itself. The series eventually dispelled a “cycle of commentary” that fixated on how she played “perfectly,” she said, denying that that was her intention.But even if Hahn sees her posts as modeling just one possible approach — practice isn’t perfect — and certainly not as lessons in how to practice or play the violin, she has come to accept what she calls their “greater purpose.” She has no plans to stop them just yet.“As a student, I never saw someone practice,” Hahn said. “I would sort of illegally listen to the wall, or even if I would poke my head into the window to see who was there, then you would duck down. You know, you tried to listen a little bit.”“We had no idea how people achieved what they achieved,” she continued, “and the fact that people have embraced the project, started doing it themselves, they’re getting comfortable posting stuff that isn’t polished — it feels like maybe the idea was mine, but the game changer is the pickup of this community.” More