Wong, 38, is one of a handful of people in Hong Kong who know the secret art of bian lian, or face changing, for which Sichuan opera is famous.

The art-form got its start earlier this century, when a Robin Hood-type character used face masks to evade capture by the 'authorities'. With a twist of the neck and a flip of the wrist, performers can leave the casual observer spellbound.

However, keepers of Sichuan's artistic traditions insist that the sleight of hand behind this feat should not be taught to women or outsiders.

Canto-pop star Andy Lau Tak-wah ran up against this prohibition recently when he attempted to become a disciple of bian lian master Peng Denghuai.

Mainland authorities initially baulked at Mr Lau's operatic ambitions but eventually relented, according to Taiwan's Central News Agency, and allowed him to enrol at the Sichuan School of Opera.

A spokesman for the Ministry of Culture's Drama Division says there is no 'official ban' on teaching bian lian to people outside the mainland and says the matter should be handled by Sichuan's Cultural Department. 'They don't have to report to us,' the spokesman told the Sunday Morning Post.

The about-face occurred after the pop star and his mentor, who holds the record for 14 changes in 25 seconds, appeared in a commercial using the skill as an analogy for speedy broadband Internet service. Suddenly, outsiders are seen as a way to keep the art form alive.

Ocean Park waited for the Lau controversy to die down before it alerted the media to Wong's work, which is performed in its Qing-style theatre only for private parties. But nothing can keep him from his beloved bian lian.

'I saw it a long time ago and loved it at once,' he says. 'It is a unique skill.'

However, as a native of Guilin it was not easy for Wong to find a willing teacher. He found one only by promising never to disclose his mentor's name or to pass on the secret.

'My instructor has broken some rules to teach me,' he says. 'I am not a Sichuan opera actor, and I am not supposed to learn the skills.'

The strict rules of bian lian would seem to be a dilemma for the art's masters.

The King of Masks, a 1996 film by director Wu Tianming, who headed the Xi'an Film Studio during its creative heyday in the 1980s, tells the heart-wrenching story of an itinerant performer in 1919 Sichuan.

The King of Masks' only son has died, and he longs for another to pass along his art. Desperate, he seeks to buy one. Dire economic conditions have led to a ready market in young children . . . all girls.

Nevertheless, he finally manages to buy Little Dog, the seven-year-old son of a peasant. Too late, the King discovers the lad is a lass.

Not unlike Wong's teacher, the King of Masks is then forced to make a difficult decision: let the art die or break tradition.

Wong says he started performing in Guilin opera at the age of 10, and that tough training helped him pick up bian lian quickly. He had his first lessons in Sichuan over 10 days in December 1998. After that, he spent six months in Hong Kong practising. Then he was ready to return to Sichuan for a second series of lessons. 'I feel so happy and satisfied to have learned this art,' he says.

Changing masks is not the only aspect of bian lian requiring great skill; being able to sew silk masks to fit every part of a performer's face is just as important.

Mr Wong relies on his teacher in Sichuan to tailor the 20 coloured masks he uses during his performances.

Professor Chan Sau-yan, the director of Chinese University's Chinese Opera Information Centre, says the colour of each mask has a meaning. 'For example, red refers to righteousness and white to villainy.'

Masks have played a crucial role in mainland opera since the Sung dynasty, when limited resources forced the small number of people involved in a production to play several roles each. Makeup was time-consuming, so the actors used masks to represent different parts.

Mr Wong says the beauty of bian lian is to portray each character's emotions, their happiness, sadness and anger. 'You cannot just swing your head and show the mask . . . expression from the eyes is most important.'

Like all bian lian performers, he must keep prying eyes from learning his art and hides himself away while getting ready for a performance. No Ocean Park staff are allowed to enter his dressing room.

Professor Chan says part of the reason bian lian is permeated by so much secrecy is because it is a major source of income. There are more than 400 varieties of opera on the mainland, keeping hundreds of thousands of people employed.

'The trade is a competitive one,' he says. 'Changing face is unique to Sichuan opera. If Sichuan performers lose the secret of it, they will lose their attraction and their income.'

Professor Chan says the tradition of not passing the art's secrets to women, though unfair, may be necessary. 'It is a balance between losing a culture and making a fair society.'

However, Wong does not quite see it the same way. 'It is discriminatory not to teach girls,' he says.

'We have a modern society now, and men and women are equal.'