A dancer in a black leotard leaps across the room, her compact body arching through the air like a Ping Pong ball. But the arm, the right arm is not quite right, it seems.

The choreographer, who has flown in two days previously to dust off this ballet that he taught the Cuban troupe three years ago, asks her to "hold this arm longer," and the leap is practiced again with a barely perceptiable change.

Brian MacDonald is the choreographer, a tall, balding man with superb carriage who gently hones and polishes the dancers' movements with a word or a demonstration. Although he was the first choreographer from a capitalist country to work with the Ballet Nacional de Cuba, he does not speak Spanish, so his wishes are communicated through translation or mime.

The company, founded and headed by ballerina Alicia Alonso, is in the midst of a three-week run at the Kennedy Center, ending July 8. Three of MacDonald's works - including one to be performed tomorrow - are in the current repertoire, so he flew down from his home in Montreal for a few days before going on to direct a play in Toronto.

McDonald is one of a small number of international free-lance choreographers. They work in many languages, in numerous countries, with artists of different backgrounds, unified by the common world of dance. The porblems they juggle, whether language or national temperament, are different from those of the resident choreographer or dance teacher.

Even politics is a factor. If it wasn't for the fact that Canada and Cuba were friendly, long before detente with the United States and the island began, MacDonald would not have met Alonso. He was included in a visit to Cuba made three years ago by former Canadian prime minister Pierre Trudeau, as a government-sponsored representative of the arts.

In Cuba, socialist politics permeate even the ballet, which attributes its existence to Fidel Castro almost as much as to Alonso.

"There's a kind of public morality in Cuba," MacDonald, said on Thursday night. "Before, there was a kind of public cynicism. Now everyone calls each other 'comrade.' There's a terrific sense of team spirit." This atitude eliminates some of the prima donna temperament that is almost a ballet cliche, he said.

"Cubans dance for the glory of Cuba," Russians dance for the glory of Russia," said MacDonald, who is familiar with both. "And Americans dance because it's a good part."

But which attitude produces better art?

"Well, American dancers are really great. They can do anything you ask. Russians are so steeped in the classical method they have a hard time adapting to modern forms. The Cubans' ability to grasp new movement is extraordinary."

MacDonald sees other differences as well. Once in Cuba MacDonald was conducting a rehearsal when his temper erupted at the chattering going on among the corps de ballet: "The whole room was stunned into silence after I exploded," he said, "and the next day every single member of the company came to me individually and apologized."

That would never have happened in America, he said. "You wouldn't have to yell out like that here."

Another time he had to send $800 worth of silk jersey from Montreal to Havana for costumes because the fabric was unobtainable there. The material was picked up at his home one morning, arrived in Cuba later that day, and he was reimbursed the$800 by the following afternoon. "There are companies in Europe who don't pay you for two years because they can't be bothered to calculate the royalties," he said. "In Cuba, they are wonderful. They don't want to be beholden to anyone."

MacDonald dates his involvement with dance from Sept. 28, 1944, when he was 16 and saw a performance by the American Ballet Theater in Montreal. He quit McGill University, took a job as a critic of music, circus and nightclubs at the Montreal Herald, and started ballet classes. He had to start at the very beginning, "with the babies," and worked his way up.

"It was very difficult for my father, who expected me to be a lawyer," he said. A few years later, while performing with his first wife at a nightclub, he smashed his arm - " the wrist concertinaed right up" - so he turned to teaching and choreographing to support his family, which by that time included a young son, who is now 25.

In the intervening years he has worked with Robert Joffrey, the Royal Winnipeg Ballet, the Royal Swedish Ballet, won a gold medal in Paris, directed television specials, acted, directed opera, and studied in New York and Leningrad, among other things. He now stays in shape with a 20-or 30-minute daily jog.

His choreography usually starts with a piece of music. "I listen to the music over and over again. I (day) dream a great deal. I tune into the subconscious; I believe it's all there . . . It's a kind of osmosis that takes place. It sounds mystical, but it gets to be rather a holy process as you get older and the ego becomes less important.

"I'm like a tailor in rehearsal, cutting out pieces of a suit. The dancers think you're mad. But then it all comes together as a whole."

The intricate combinations of steps, turns and attitudes that make up MacDonald's dances are preserved by videotape, according to a clause he has written into his contracts. Otherwise, like the old ballets, they disappear with the memories of those that create or dance them.

Interest in dance has increased tremendously in recent years, and audiences seem not only supportive but almost rapturous, as though dance taps a vein of human expression that no other form does.

"One reason is that we've stripped down the body," MacDonald said. "We're looking at primitive man, in a way. It's basic. Also, it's very sexy."

The Cubans, who seem to be shorter and more curvaceous (even the men) than the skinny, pale dancers more familiar here, add support to the idea that dance is sexy. It also is a theme that flows through some of MacDonald's dances.

Prologue For a Tragedy," which was performed Thursday night and will be seen again on Sunday, is a kind of mating dance set after the wedding of Othello and Desdemona. "Time out of Mind," originally choreographed for the Joffrey Ballet, grew out of MacDonald's realization after his first wife's death "that one does not lose sexual desire in the face of death, it is so strongly in our genes." The third dance, "Remembrance," is a pas de deux for Alonso and her young dance partner, Jorge Esquivel.

"Brecht once said that the theater teaches us all we need to know to survive," MacDonald said. "In the compression of what happens at 8:30 are the lessons of all the vicissitudes of life.

"Of course, it's also a job." CAPTION: Picture 1, Choreographer Brian MacDonald, by Harry Naltchayan - The Washington Post; Picture 2, Brian MacDonald, by Harry Naltchayan - The Washington Post