WHAT is Jackie Chan doing on the rooftop of a steam locomotive hurtling through a corn field? Why is Tsui Hark in the Forbidden City surrounded by 2,000 lion dancers? Or Michelle Yeoh atop a Ming Dynasty funeral pyre? They are part of a cinematic exodus to the People's Republic of China, the result of an open door through which Hong Kong-originated winds of change are already beginning to transform the face of Mainland Chinese cinema. At the current rate that movies are produced in Hong Kong, some 500 features will be completed between now and the territory's reversion to Chinese rule on July 1, 1997. And if the current trend continues, a growing percentage of these comedies, kung-fun pictures, and costume epics will be filmed across the border. ''In terms of movies, '1997' will be more a case of Hong Kong affecting China than the other way around, particularly in the areas of production and distribution,'' says Chan Pak-sang, editor-in-chief of City Entertainment, Hong Kong's leading film journal. ''Over 20 Hong Kong film crews have gone up north to shoot this year so far, and it is inevitable that this continued exposure to Hong Kong technique will have a cumulative effect on Chinese technique.'' W. D. Yang, a Beijing production co-ordinator, concurs. ''The Hong Kong crews come here with their advanced equipment, cranes and cameras that Chinese studios cannot afford at present. But they make us aware of these advancements, things which some of the older film-makers resist but which the new generation of movie technicians are eager to try.'' Ann Hui, a Hong Kong director who produced the Qing-era martial arts saga Fong Sai Yuk in Beijing, noted a keen interest in China on the way her crew executed the action sequences. ''They very definitely use Hong Kong movies as a model, especially the way the shots are set up and the manner in which stunts are done.'' But not all Chinese film-makers regard Hong Kong movies as a learning vehicle. ''Sure, Mainland studios and crews like working on Hong Kong productions,'' says film director Xie Fei, former principal of the Beijing Film Academy. ''But not necessarily for the educational opportunities. It's mostly a matter of money. The remuneration is so much higher that local productions were sometimes finding it hard to book studio facilities and personnel. ''So starting this year the authorities have instituted new regulations, restricting the number of non-PRC productions to an annual total of 30.'' Impossible to restrict, however, is the enthusiasm of the Chinese public for Hong Kong movies. ''There's a huge audience there,'' Xie Fei affirms. ''Mostly young people or those with a relatively lower level of education. ''The high school crowd wants to see the Hong Kong stars, like Andy Lau and Leon Lai. There really aren't any Mainland superstars nowadays, except Gong Li. and she only became celebrated here after she was 'discovered' in the West.'' MOST Hong Kong movies are available only in pirated versions. ''The rights to these pictures are too expensive for legal distribution,'' explains W.D. Yang. ''Most are shown in video parlours, and on the billboard they'll change the film's name to thinly disguise the fact that it's pirated. ''The main drawing card isn't so much the movie itself as the genre and the stars. An action picture with Chow Yun-fatt is sure to grab a huge audience, no matter if it's one of his better pictures or not.'' Though piracy is rampant, there is also a growing number of Hong Kong movies being legally distributed throughout China. ''In the case of co-productions between Hong Kong and specific Chinese studios, the PRC distribution rights are usually assigned, or sold outright, to the Chinese side,'' says Chan. The most successful example to date is Dragon Inn, co-produced with Hunan's Xiaoxiang Studio and whose stars include Hong Kong's Tony Leung Ka-fai and Taiwan's Joe Wang Cho-yin and Lam Ching-ha. The popularity of Hong Kong action films has induced many studios to try to make home-grown versions. ''Especially kung-fu and cops-and-robbers movies. But there isn't any money to invite Hong Kong stars, so there's no way they can really compete in the market place,'' states Xie Fei. ''Even if we could, a major market - Taiwan - is cut out, because under current Taiwanese regulations, movies made by Mainland directors cannot be released in Taiwan.'' ''That's why Zhang Yimou's movies have still not been officially shown in Taipei. But I think this regulation will change fairly soon,'' says Chan. Zhang Yimou is one of the few Mainland directors whose fame has extended to Hong Kong, but even such Academy Award-nominated films as Judou and Raise the Red Lantern fail to find too broad an audience in the territory. Chan explains, ''Hong Kong audiences aren't too interested in non-action-oriented themes. And they have a lot of trouble relating to the Chinese countryside. Zhang Yimou's Story of Qiuju did very poorly at the box office, despite the presence of a big star like Gong Li.'' The highest grossing Mainland film in Hong Kong's history is Judou, whose $8 million in ticket sales ranked only 51st overall for 1990. Xie Fei concedes that ''Chinese films don't exert much influence in Hong Kong.'' Chan agrees. ''The main affect of a director like Zhang Yimou isn't so much that he makes Hong Kong directors want to make quality artistic films as it is to implant in them a desire to build up a reputation overseas and win awards.'' ''One aspect of the Hong Kong film industry that could certainly benefit from Mainland influence is that of the importance given to scripts,'' states Chan. ''It's not unusual for a movie to commence shooting in Hong Kong before a script is completed, a situation that would never happen in China.'' Part of the reason for this is that in China, screenplays must undergo official scrutiny before a project is approved. It is a tradition that looms ominously on the Hong Kong horizon. ''The biggest affect of 1997 will be on the content of Hong Kong films,'' claims Chan. ''No more movies like Her Fatal Ways,'' a political satire about a thick-headed communist cadre, played by Dodo Cheng, and the clash of cultures that ensure when she is sent on assignment to Hong Kong. The comedy was a popular and critical success, leading to two sequels. Cheng was named Best Actress at the Hong Kong Film Awards, and the movie ranked number 9 at the box office in 1990. All this despite the refusal by the territory's PRC-affiliated theatres to screen the film in their venues. But at least Her Fatal Ways could be produced and released. ''After 1997, who knows?'' asks Chan.