When Brittany Janae Newell started as a model in 2010 after graduating from high school, she understood the grueling schedule both on and off camera. The fitness and beauty routines came easily enough and she enjoyed the e-commerce and commercial work, she said. But five years ago, when she received a callback from the director of a shoot for PayPal, her first big-name client, everything changed.

“The director loved my hands and asked me to come back the following day to work with him as a hand model,” said Ms. Newell, 29, who is based in Los Angeles. “I was like ‘A what model?’”

The following day, she “showed up in pajamas, got a manicure, worked for a couple of hours and got paid $1,000,” Ms. Newell said, adding, “I was so shocked. I didn’t want to even be a regular model anymore.”

Today, Ms. Newell, who describes herself as half Black, half Eastern Indian “with a golden brown skin tone on the darker scale and whose hands kind of resemble Zoe Saldana,” no longer arrives to photography shoots in sleepwear (although athleisure is fine).

With 90 percent of her modeling jobs calling for hand modeling and having booked clients like Fitbit, Samsung and LG Watch Style, she said she has come to realize the preparation and sacrifice necessary to remain in demand in the professional world known as parts modeling.

Parts modeling — as in body parts like hands, wrists and arms — is a vital piece of the watchmaking industry. Once a timepiece is created, it is often up to parts models to help draw attention to it through advertisements, editorial shoots in fashion magazines or video campaigns, for example.

“To get that certain shot, your whole body is working just for your hand,” Ms. Newell said. “Your fingers have to look relaxed; you have to make sure the key points of the watch never lose connection with the camera.” Her wrist and hand movements have to be realistic, she said, and a model has to be able to pivot from one item to the next, as a lot of different products are photographed in succession.

Dani Korwin, president of Parts Models, an agency in New York, explained further: “The entire hand, a part of the arm, the wrist — these models use a part of themselves to make that watch the sexiest and most desirable it can look.

“You visualize that watch on your own wrist because a wrist or an arm is selling you the product,” she said in a phone interview. “Physiologically, parts models are relaying that desirability to the consumer.”

Ms. Korwin, a former hand model herself, said she started her agency in 1986 and now represented more than 200 male and female parts models. She emphasized that as a watch wearer herself (that day, a Gerald Genta Mickey Mouse watch), “I understand and appreciate good watchmaking, and also what it takes to sell them.”

Although parts modeling is not a new aspect of the industry, it has experienced notable shifts, Ms. Korwin said. In recent years there has been a “noticeable uptick for people of color,” she said, with her clients seeking a range of skin tones for their projects.

And while social media forced a change in the production and distribution of content, it “hasn’t negatively impacted my business,” she said.

Johnny Tyronne, a 42-year-old model who lives in the Astoria neighborhood of New York, said he “didn’t take parts modeling seriously” at first.

“I told my friend Joe to take a photo of my hands holding some martial art weapons that I had in the trunk of my car,” he said in a phone interview. “Now I can spend 12 to 14 hours a day” on a set and “pay the bills, mostly doing hand modeling.”

Day rates for hand models can range from $150 to $1,500 for editorial shoots and from $250 to as much as $4,000 for commercial work, which is comparable to rates for typical full-body models.

Mr. Tyronne said he started modeling 15 years ago, adding hand modeling 12 years ago, and is now represented by two parts-modeling agencies in New York: Parts Models for print work and Carmen’s Hand Models for television commercials. Mr. Tyronne lists his shirt, suit, waist, inseam, shoe, glove and ring size on his Instagram feed, using the social media site as a real-time résumé.

His wrist has displayed watches from IWC, Baume & Mercier, Patek Philippe and Longines in ads for clients such as Saks Fifth Avenue and the watch retailer Tourneau, and his hands have doubled for those of the singer John Mayer in ads for laundry products.

“Modeling watches is pretty straightforward until it isn’t,” Mr. Tyronne said. “The security guys on set are like ‘You can’t go to the bathroom; you can’t leave the room with that watch on,’ so that took some adjusting. And I walk around set with my hands in the air. It gets the blood to flow down from the veins so when the hands are photographed, the veins don’t show as much.”

To maintain his money makers, Mr. Tyronne wears gloves when doing anything that could damage his hands, which includes ax throwing and washing dishes: “A little scratch is fine, breaking a nail is not.” He also moisturizes “a lot” and carries Dermablend powder and concealer with him.

Another hand model based in New York,  Antonia O’Donoghue, 35, said she understood the regimen. “I’ve been called day of, hour of, and asked to come in because the model they first booked had fake nails on or couldn’t model a watch,” she said. “I am always ready.”

Ms. O’Donoghue’s preparations include getting two or three manicures a week, keeping lotion on each table in her home and in every handbag and having a to-go kit that includes a nail file, cuticle oil, a little glue and a tea bag. “The tea bag trick is essential to fix a split nail,” she explained. “You cut a little piece and put it over the split, add a little glue, some topcoat and you file it. It works.”  

Her first hand model editorial job, a full-page photo of her holding a bunch of doughnuts and cupcakes, ran 18 years ago. “I’ve been doing parts modeling since I was a kid going into college,” she said.

Through an agent at Parts Models, Ms. O’Donoghue has worn Breguet, Chanel, Rolex and Versace watches, both for watch-specific shoots and for fine jewelry projects.

“There is a need for good-looking hands, and a lot of work for that,” Ms. O’Donoghue said. When asked if she ever considered other parts-modeling, she responded, “Oh dear God, you don’t want to see my feet.”