But off the record they can be much more forthcoming - especially with information that might harm a rival and not themselves, said Cheung, a triad expert with the police criminal intelligence bureau.

'When police officers interview triad members during an investigation, they normally remain silent under caution but are very willing to tell the whole story when they are interviewed not under caution,' he told the Court of First Instance yesterday.

Triad informants would tip off police officers about matters concerning other triad branches and matters that did not concern themselves, he said, describing this as a 'special kind of relationship' between the police and triad members.

Cheung was giving evidence as an expert witness in the Court of First Instance trial of five alleged Wo Hop To triad members, accused of involvement in a plot to murder casino dealer Wong Kam-ming in May, last year, after a one of his clients won hundreds of millions of dollars in Macau casinos.

Cheung told the court that the history of triad societies could be traced back to 1674 when an organisation was founded in China with a mission to overthrow the Qing dynasty.

'In Hong Kong, triad societies only engage in illegal activities,' he said. 'The sole purpose is to make money. [Triad people] do illegal things, and they do legal things through illegal means.'

Giving a rare public insight into the alleged activities of some of the city's most notorious gangs, Cheung said Wo Hop To was one of the most influential, with members active in Western District, Aberdeen, Wan Chai and Yuen Long. Its activities included loan sharking and gambling.

Another society, Wo Shing Wo, was active in Kowloon West with underground casinos, drug trafficking, piracy, extortion and vice.

The 14K society, he said, was involved in extortion, prostitution, piracy and illegal franchising of taxi stops.

Sun Yee On, active in the Tsim Sha Tsui area, engaged in activities related to dangerous drugs, extortion and piracy.

Cheung said there were 2,378 triad-related cases last year, with 648 involving woundings and serious assaults, compared to 2,258 and 555 respectively in 2007.

Of the violent crimes, most arose from fights in public places and were related to conflicts between different triad societies.

Under a code of 'loyalty and unity', triad members usually obeyed without question any instructions from higher levels of the society's hierarchy.

'Most of the time they simply follow instructions from their superiors without knowing much about the background,' Cheung told the court.

The experienced triad-fighter said the lowest tier comprised two categories: 'ordinary members', who had been through a formal ceremony, and 'hanging the blue lantern' members, who had not.

Basic members could be promoted to various kinds of office bearers, including 'the red pole', also known as 426, who traditionally took care of the operations of the society; 'the white paper fan', or 415, who was responsible for the administration and organising ceremonies; and the 'grass sandals', or 432, who communicated between different branches.

Office bearers were all qualified to be leaders of a branch and had control over money, according to the inspector. Faction leaders were called 'big brothers' or 'Dai Gor', while the top leader was 'Ah Kung', or 'grandfather', the court heard.

In the case, before Mrs Justice Verina Bokhary, See Wah-lun, 30, Tang Ka-man, 31, Wong Chi-man, 26, Yeung Chun-kit, 22, and Chan Ho-leung, 35, have pleaded not guilty to charges including acting as triad members and conspiracy to cause grievous bodily harm.

See, an alleged senior member of the group, is also charged with conspiring with 'Tsang Pau', identified in court as Macau casino operator Cheung Chi-tai, to commit murder, and soliciting nine people, including the four other defendants, to murder.

The hearing continues today.