It is not his charm that will disarm you, but a strength that no science can explain.

At 78, wing chun grandmaster Chu Shong-tin is a soft-spoken man of slight build, but those who know him will speak of a physical power they have never witnessed before.

“I felt power from what seemed to be an old, frail man,” said Nima Khezrnejat, who moved from Australia to Hong Kong for the Chinese martial art and has opened a wing chun gym in Central.

The 27-year-old described Chu’s power as “something that you only hear about in movies or books”. He trains at Chu’s studio in Sham Shui Po, in an old residential building on Cheung Sha Wan Road.

Chu, who trained under the legendary martial arts teacher Ip Man for 14 years and lived with him for five years, teaches three weeks out of every month. There is no formal structure to the class, attendance is by invitation only and there is a communal dinner break, which creates a sense of family among the students.

His journey into this concept-based form of self-defence started in the most ordinary of settings in Hong Kong – a restaurant – but through an extraordinary teacher. Chu was 17 and working as a secretary at a restaurant that Ip, who also taught Bruce Lee, used for classes. “I just observed the classes,” Chu said.

But wing chun struck a chord, and Chu soon became a student.

“When I trained under Ip Man, I was young, my body was thin and my health was not good.” On Ip’s advice, he took up the relaxation style of wing chun, as opposed to one based on muscular force.

Chu’s 37-year-old son Horace said the sport had had a profound effect on his father’s life. Chu was diagnosed with late-stage cancer and told he did not have long to live. But since that prognosis, Chu has recovered, and doctors have been unable to explain what happened.

“The theory of his wing chun style is impossible to explain with modern science, by physics,” his son said.

Once Chu Shong-tin briefly met Lee. The man who later became a movie star had asked Chu for help on building a dummy to practise wing chun. Chu described Lee, who died in 1973, as “clever and active”.

In Chu’s 40-odd years of teaching, he has guided many students, including Sebastian Soza, 30, who says it is not easy to grasp wing chun. “You’re trying to build the intention of the movement and until you feel it, that makes the difference.”