TAIWAN'S handling of yesterday's hijacking reveals just how much posturing is still involved in the Kuomintang's determination to keep its distance from China. The two hijackers are to be tried in Taiwan rather than repatriated to presumably face a harsher fate in courts on the mainland. Moreover, to maintain the fiction of refusing all direct air links with the mainland, the pilot who flew the plane back to China was asked to land in Hongkong - a request he subs At first sight, there is little difference between the details of yesterday's incident and Taiwan's handling of the previous hijacking in May 1988. During that incident the plane was returned to China after just six hours, despite frostier relations thenbetween the two rival governments. The move was then rightly taken as a sign that ties were about to improve. The two hijackers were detained in Taiwan and sentenced to 31/2 years' imprisonment and granted asylum. The pilot of the hijacked Boeing 737 was told to stop in Hongkong on his way home, but failed to do so. It is almost as if nothing has changed in five years. Yet, as has so often been the case in Taipei-Beijing relations since the death of former Taiwanese President Chiang Ching-kuo in early 1988, appearances are deceptive. In the interim, Taiwan's ''three nos'' policy of no contact, no negotiation and no compromise with the mainland has gradually been confined to history. The ban on direct air links is one of the few major relics of that policy still in force. It has been maintained in the hope that it can be used as leverage to get Beijing to relax its opposition to Taiwan's re-entry into international organisations, especially the United Nations. Today the KMT government's senior negotiator with the mainland, Mr Cheyne Chu, director of the semi-official Straits Exchange Foundation (SEF), will visit Beijing to prepare the groundwork for an unprecedentedly high-level meeting in Singapore later thismonth. There SEF Chairman Mr Koo Chen-fu and his mainland counterpart Mr Wang Daohan, the Chairman of the similarly semi-official Association for Relations Across the Taiwan Strait (ARATS) and a former Mayor of Shanghai will focus on ways to fight crime and smuggling and slow the influx of illegal immigrants from the mainland into Taiwan. The two sides already have a semi-official extradition agreement - although hijackers are not included - and co-operate loosely in crime prevention. However, there is still a great deal more to be done and a tougher commitment from Beijing to crack down on smuggling and illegal emigration would also benefit Hongkong. If Beijing can take a firm line with the authorities in Fujian from which much of the illegal immigration flows to Taiwan, it might prompt the demand that Guangdong, too, rein in the lax and possibly corrupt officials thought to be involved in similar illegal activities. More directly, however, it would be an important signal to both hijackers and criminals alike if the Taiwanese courts were to impose stiff sentences on yesterday's hijackers and deliberate very carefully before offering them political asylum. International law frowns sternly on air piracy. Neither China nor Taiwan can continue to behave as if hijacking from their rivals across the Strait is acceptable under any circumstances.