uen Kay-San (Ruan Qishan) was born in 1889 in Foshan, Guangdong. Being the fifth child of his family, he was commonly called Yuen Lo-Jia (Yuen the Fifth). At a very young age, Yuen Kay-San and his elder brother, Yuen Chai-Wan (Ruan Jiyun) demonstrated great interest in and aptitude for the martial arts. Luckily, their father was a wealthy fireworks merchant and could afford the very high price wing chun kuen masters at the time commanded for their instruction. Wanting to encourage his sons, he spent a small fortune to engage the renowned Fok Bo-Chuen in order to teach them the skills of wing chun kuen.

Yuen Kay-San studied under Fok Bo-Chuen intensely for many years. Eventually, through hard work and determination, he learned all Fok Bo-Chuen had to teach (including the fists, the dummies, the pole, the knives, and the darts). Yuen Kay-San also succeeded in developing the devastating power of the iron sand palm. Although he had developed a phenomenal foundation under the guidance of Fok , he was still eager for more knowledge. This quest eventually led him to Fung Siu-Ching.

Initially, Fung Siu-Ching felt that there was little he could do to improve Yuen's wing chun kuen but later had a change of heart when he saw the young man's dedication. Already accomplished in the wing chun of Fok Bo-Chuen, Yuen Kay-San's education under Fung consisted of advanced application and close-body fighting (including seizing & holding, counter seizing & holding, finger breaking, etc.). Yuen was also said to have honed his dummy, pole, and flying dart techniques under Fung's guidance.

Following their training with Fung Siu-Ching, the Yuen brothers took different paths. In the 1930s, Yuen Chai-Wan moved to Vietnam where he taught Wing Chun at the Nanhai and Shunde Expatriates Association. Many branches of Vietnamese wing chun (romanized vinh xuan in Vietnam), both in Hanoi and later Saigon, recognize Yuen Chai-Wan (Nguyen Te-Cong) as their founder.

Yuen Kay-San stayed in Foshan and worked on developing his wing chun. Throughout his lessons, Yuen had always taken copious notes. He spent time analyzing the scientific principles of wing chun kuen and became one of the first to document its formal concepts. Linking together and refining all the knowledge he had acquired, he developed a complete understanding of wing chun and went on to found remarkable methods and principles encompassing its forms and functions. Yuen Kay-San did not boast of his skills or seek out confrontation. Instead, he used his abilities only to defend himself, his property, and for practice. Although he did everything possible to avoid it, from time to time circumstances arose which forced Yuen into combat. Many of these matches became the subjects of articles written by local journalists such as Ngao Soi-Jee, the famed author of Foshan Jan Sin-Sang (Mr. Jan of Foshan).

A deeply private man, Yuen Kay-San tried as much as possible to keep his knowledge of Wing Chun to himself. He did not open a school, engage in public displays, and did not even reveal his background to acquaintances. Eventually, Yuen Kay-San was introduced to his only disciple, Sum Nung, who at the time was learning from a man named Cheung Bo.

Cheung Bo (Zhang Bao), known as Dai Ngao Bo (Big Bull Bo) although almost a decade younger, was one of Yuen Kay-San's close friends. Cheung worked as a chef at Tien Hoi, a local restaurant next to Kuaizi Jie (Chopstick Street). A large and powerful man, he taught Wing Chun kuen to a small group of fellow staff members at night when the establishment was closed.

Cheung Bo learned his Wing Chun from Nationalist army doctor Wai Yuk-Sang (Wai Yusheng), who studied under Fung Siu-Ching's pupil Ngao Shi (Ou Shi).

Cheung Bo was renowned for his fighting skills, which he reportedly used to keep even the raucous young martial artists who frequented the restaurant in line, and for his talent with the gold-coin darts.