Merce Cunningham was the greatest “acquired taste” in modern dance. And living to the active old age of 90, with 60+ years in the public eye, we had a lot of time to acquire that taste.

His choreography is revived, explained and dissected in “If the Dancer Dances,” a revealing documentary about reviving his 1968 piece “Rainforest,” complete with the “Andy Warhol aluminum pillows” balloons), David Tudor music and ripped Jasper Johns “nude” costumes.

New York choreographer Stephen Petronio unleashes his company on one of Cunningham’s classics. But he doesn’t trust himself to stage it.

“The beauty…and amazing thing about dance is that it gets passed from one body, one soul, to another,” Petronio explains. “It comes out of the body, it goes into the air. And then it disappears…How do we keep their work alive?”

We convince the dances who worked with Cunningham — among them Andrea Weber and Meg Harper, to teach the work, the style, to young dancers in a company whose works about “constant motion.”

“There’s going to have to work from a different place within their bodies,” Weber notes. “Everything everything EVERYthing comes from the back!”

He’d build his company, and rebuild it around certain types of dancers, and certain physical types.

“Merce,” who starred in his own pieces late into life, “had these crazy-long arms and legs,” Weber says to Petronio’s troupe, emphasizing the difficulties that creates. Every body part is vital, every muscle just so.

“I’m going to HOUND you about the hands.”

“Rainforest,” a work of graceful, halting lurching, leaps, embraces, blooming and stalking, was inspired by Cunningham’s recollection of a rainforest near where he grew up in Washington state.

Backstage, the Gino Grenek, dancing the lead, gripes that “Everything is cramping,” thanks to the en pointe poses held impossibly long, legs or arms suspended in air for seconds upon seconds, the contortions demanded of the human back.

No wonder a young dancer in the company, Nick Sciscione, whispers “The name evokes FEAR.”

“If te Dancer Dances” — the film takes its title from a Cunningham truism, “If the dancer dances, everything is there!” — underlines what Petronio used as his reasoning for bringing in as many Cunningham vets as possible for the rehearsals. “You can’t teach it from the written page, can’t really learn the dance from a video,” Petronio says. It’s all about “muscle memory,” practiced and passed on, “dancer to dancer.”

If Cunningham’s work is to survive, it will be through efforts like this (The Cunningham Trust was set up to ensure that.).

Cunningham veteran Gus Solomons jr analyzes the essence of Cunningham’s art, “Shapes at different speeds, very fast, or very slow,” he says. “Stripped down abstraction” that can “approach physical impossibility. And that’s what’s exciting.”

Yes. “Everything cramps” performing his works.

All this explanation is useful to the viewer, casual dance fan or otherwise, because for all the glories of lithe, sweaty bodies moving in intense physical concentration, of seeing leads Dava (Davalois) Fearon and Grenek and others mastering the movements, no film version of Cunningham’s work can overcome that “acquired taste” thing in itself.

But “If the Dancer Dances” piques the interest and widens the appreciation for just what a test these pieces are, even to the performing arts world’s Olympic-level athletes — dancers.

MPAA Rating: unrated, some profanity

Cast: Davalois Fearon, Gino Grenek, Meg Harper, Nick Siscione, Andrea Weber, Stephen Petronio

Credits:Directed by Lise Friedman and Maia Wechsler. A Monument release.

Running time: 1:26