I have heard 3 slightly different versions of this story. They differ only in how the fight came to take place, however, and the fight itself is always told the same. This version is the one my teacher heard. It states that Yu Lok-Gong was challenged by another guy who didn't show up and so he decided to fight Yuen Kay San instead. Another version states that Yip Man originally challenged Yu Lok-Gong but had to leave for Hong Kong before it could take place, so he asked Yuen Kay San to fight in his place. The third states merely that Yu Lok-Gong waited for Yuen Kay-San in ambush and attacked him while Yuen was on his bed. A version of the story was published once in "Wing Chun Kuen Wong Jan Sin San," a (poorly?) written Chinese novel (well, martial soap-opera) allegedly about the life of Leung Jan. Buried therein is the author's account of Yuen "Lo Jia" (the Fifth) and Yu Lok-Gong.
Anyway, here's the story as I remember hearing it:
Yu Lok Gong of the Mantis style was sent to Foshan from the North by his famous teacher, Law Kwan Yuk, to take control of that town's branch of the Jing Mo Association. Upon his arrival, regional pride soon caused tempers to flare. Challenges were made and eventually a match was set and a time and place chosen.
On the day of the fight, Yuen Kay San came to observe and sat near-by, shoes dangling from his feet. When the fight was to begin, however, Yu Lok Gong's opponent was nowhere to be found. Yu Lok Gong had heard that Yuen Kay San was a Wing Chun practitioner of considerable skill and soon turned his attention towards him. Yuen Kay San, not interested in such contests, begged off but Yu Lok Gong kept insisting and eventually launched an attack at Yuen Kay San. Forced to defend himself, Yuen Kay San rose quickly to his feet, without time to even place his shoes (thus forced to wear them slipper-style) and dissolved the attack. Yu Lok Gong was renowned for his footwork and devastating leg techniques (it was said he could send the 100 lbs+ rice bags flying across the room with a single kick), yet Yuen Kay San pressed him back, sticking to his movements and jamming his famous legs. Yuen Kay San continued to press until Yu Lok Gong hit his back against a barred door and he could no longer retreat. At Yuen Kay San's mercy, Yu Lok Gong was forced to submit. Yu Lok Gong, impressed by Yuen Kay San's superb technique, said that "Wing Chun Arms and Mantis Legs" would be a powerful system.
He was thereafter quickly recalled to the north.