It was well known at the time that Yuen Kay Shan and his brother Yuen Chai Wan only knew Siu lin tao, some san sik techniques and bamboo dummy before they started hanging out at Ng Chung So's school... Later in Foshan, yip Man specifically told his student Kwok fu not to practice the wooden dummy form in front of Yuen Kay Shan being concerned that YKS would steal his wooden dummy techniques.
Interesting! What is the source of this information?