It was a hot July afternoon, and the large Upper East Side yoga studio was only half full when a student placed her mat in a wide-open space. Walking with purpose, the slim instructor grabbed the student’s mat and pointed to a new spot. “I’m nearsighted,’’ said the student, only to have the teacher grab her towel. “Why is this in the middle of the floor?’’ she demanded, nary a hint of Zen calm in her voice.

But when the class began, the instructor slipped into a new persona, softening her voice and adjusting people in the room as she guided them through their asanas.

“She definitely has an on-and-off switch,’’ says one of her colleagues.

And so it goes with bogus “gurus” all over the city — Jekyll-and-Hyde yogis who yo-yo between their pseudospiritual selves and their sincerely mean spirits.

“There is one yoga teacher who also teaches spin, which I find funny, because one is focused on calming and the other has a goal of revving you up,’’ says Financial District resident Katie Chatzopoulos, 41, who works in finance and practices yoga at Equinox, Kula Yoga Project and Twisted Trunk Yoga. “You are supposed to open up your body, so trying to do 15 chaturangas [a low plank pose] in under a minute is ridiculous.’’

It makes sense that sham opportunists are on the rise. The yoga phenomenon has grown from a spiritual practice to big business, incorporating outsize personalities, clothing lines and Instagram followings. According to a study by Yoga Journal and Yoga Alliance, about 37 million Americans are practicing yoga in 2016, up from 20.4 million in 2012, and they spent about $16 billion on clothing, classes and gear in the last year, up from $10 billion just four years ago.

While spurious yogis don’t usually approach the scary levels of Alison Dadow — one of the “yoga twins’’ whose fight with her identical twin sister, Ann, caused a car crash resulting in Ann’s death — some are ill-equipped to transport us to a more evolved state. Many New York yoga instructors only attend the minimum 200-hour certification program before being turned loose on studios across the city (some spaces, like Jivamukti, require more training). Their degrees don’t reflect the meditative skills and spiritual guidance associated with genuine nirvana trainers.

“Recently, a 26-year-old girl who was in an LA dance troupe moved to New York to become a yoga instructor. She would smoke outside, and any time there was an audition, she would ditch us. It was all about her,” says Scott Weiss, owner of Bodhizone, a physical therapy center on East 23rd Street that incorporates yoga. The teacher eventually left of her own accord.

He also recalls hiring a woman in her early 40s who had a master’s degree in exercise science, as well as yoga training, who turned out to have an air of self-importance. “She would say, ‘Oh no, no, you are not doing it right! Look at me!’ She was so conceited and walked around like she was the master, without any humility. If you couldn’t do a technique, she was rude, and instead of saying, ‘Good effort, you are almost there,’ or encouraging someone, she would say, ‘You are not getting it.’ Teachers need to have an emotional quotient. [We] finally had to let her go.’’

Then there are Instagram-obsessed yogis, whose practice of posing on beaches at sunset has become a much-mocked narcissistic cliché. “The whole Instagram phenomenon is about showing off and making other people feel bad about themselves,’’ says Cat Reinhard, a 34-year-old East Village marketing executive. “Yoga is supposed to be about looking inward and having a mind-body connection, not about posing.’’

Self-serving behavior isn’t the only quality getting New Yorkers bent out of shape. There has been no shortage of sexual-harassment lawsuits against teachers accused of abusing their lofty positions, including one at Union Square’s popular Jivamukti studio, where a woman sued her female instructor in February for $1.6 million, a case that has yet to be settled. (Jivamukti did not respond to requests for comment.)

Ali Valdez, who started her yoga career in New York before moving to Seattle to launch the online yoga channel Sattva, says abusive yogis are rampant. “People think these teachers who are charismatic and radiate health can solve their problems,’’ says the 45-year-old. “You have a very diluted 200-hour training, and these people often don’t have the knowledge, philosophy or emotional maturity to manage these situations. There are others who use yoga as fertile ground to take advantage of people.’’

‘Yoga is supposed to be about looking inward and having a mind-body connection, not about posing.’ - Cat Reinhard

Some uncomplicated practices make it easy to spot a scam artist.

Adam Greenfeld, a 32-year-old Upper West Sider and longtime practitioner who is an executive at PivotDesk, a commercial real estate marketplace, says he judges teachers more by the way they act after class than during. “You can tell the most by how they behave when people go up and ask questions,’’ he says. “Are they giving them the time of day and continuing to invest in them as people?

“Some of them just snap into character, use yoga jargon to prove they know Sanskrit and show off their poses.’’

Supermodel Nina Agdal looks sexy practicing Yacht Yoga in Miami: