Margot Robbie’s trainer has revealed the surprise No.1 weapon in the Oscar nominee’s exercise arsenal.

Robbie, who showcased her fit figure in a black one-piece on a Costa Rican beach holiday this month, follows a strict program of cardio, Pilates and ballet.

And the one move that the 28-year-old Queenslander swears by is the deceptively simple ballet jump.

The secret was spilled by Robbie’s trainer Andie Hecker, who told Who magazine ballet jumps “are surprisingly the most challenging form of cardio I’ve ever come across”.

The star, who said in 2016 Hecker’s workouts were her “go-to”, also mixes in “non-bulking cardio”, including jump rope and low-impact workouts on a mini trampoline.

Training for 2016’s Suicide Squad, Robbie’s workouts were three hours long, and featured “heavy-weighted, low reps of arabesque pulls hooked up to resistance pulleys, as well as arabesque lifts with heavy ankle weights to lift and build the butt”, Hecker said.

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But the basic ballet jump, according to Hecker, is the bomb.

So what are they, and how do they stack up against other more vaunted exercises like pushups?

“Ballet jumps burn,” Dr Josie Daw, a Melbourne ballet teacher for 25 years and whose doctorate is in contemporary dance, told The New Daily.

“They’re extremely effective. They’re a classic, high-intensity exercise and you get really fit doing them.”

Dr Daw explained how to perform a ballet jump – called a sauté – but advised going to a ballet class to learn proper technique.

“Imagine that you’ve got your legs together, toes pointed forward, feet flat on the ground,” she said.

“Then open your toes to point outwards, keeping your heels together. Bend your knees into a demi plie. You can’t go far because your heels are on the ground.

“Then push up from there into a little spring, and land in the same position that you started. That’s the most basic jump.”

If you want to try the sauté at home, she recommends doing two lots of eight jumps, having a short break, then repeating three times.

“Every ballet class ends with jumps, for maybe 10 minutes, in a 1½-hour class,” said Dr Daw.

Robbie’s regimen sees her doing the two-legged and single-leg jumps, according to Hecker.

For the single-leg variation, “start in the sauté position, then jump in the air and land on one leg”, Dr Daw said.

“You can jump higher, and you would maybe do that as a second piece of work after your sautés at the end of a class. It’s more intense because you’re taking more weight on that one leg.”

The jumps are so effective because ballet requires holding your body in a very particular way, Dr Daw said.

“When you land you can’t be slovenly. You have to land softly, beautifully, precisely, with a level of control you don’t get in other forms of exercise.

“It’s bloody hard.”

The jumps work “inner thigh muscles, calves, gluteals, the entire leg. You have to drive your belly button to the back of your spine and maintain that hold so you’re pretty much working every muscle”, Dr Daw said.

So are they more effective than pushups or planks?

“It’s a different thing because they’re pure cardio.”

Robbie, who holidayed in May in Morocco with her assistant director husband Tom Ackerley, will be seen next in movie Mary Queen of Scots, co-starring Saoirse Ronan, due for release in December.

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She has also been cast alongside her Wolf of Wall Street leading man Leonardo DiCaprio in Once Upon a Time in Hollywood, Quentin Tarantino’s movie about the Manson family murders.