There is no Chi Gerk with Hawkins Cheung either. I also know that William Cheung and Wong Shun Leung did not have it. Koo Sang mentioned it when I trained with him in 1987. When I visited Lo Man Kam and Tsui Sheung Tien in 1987, they did not have the exercise.
I think it is a method developed in the 1960’s with Ng Chan, Moy Yat, Ho Kam Ming, Koo Sang, etc.
IMO, it should not be an exercise to stay up on one leg, but rather, a dynamic method of trapping the knees, breaking balance, guiding kicks, shaving legs, kicking the support leg, push horse, deflection, striking with the knees, etc.
The way I teach it is we start in a parallel or cross leg and when we feel an opponent’s kick or leg raise, we move with it. It becomes a dynamic exercise that can be added to Chi Sao or San Sao. Without this exercise, people get “loose” about low leg defense…
Chi Gerk/Sticking Legs Categories include:
Stepping – Moving the stance
Trapping – Trapping the opponent’s leg
Striking – Use of the foot, knee, shin, thigh
Intercepting – cutting off the opponent’s attack
Running – going away from the force
Sweeping – Destroying the opponent’s balance
Sticking – Moving with the opponent
Asking – Inquiring the next move
Protecting – Guarding the body and placement of the knee
The above is how I categorize skills, I do not teach these as techniques. These are my categorization and progressions, and not anyone else’s.
Augustine Fong did run through his system with me when he visited NYC in the 80’s. I combined what I learned from him and from my other instructors and have this method now. Probably Augustine was the first to really show his system in depth. I admire his innovation and teachings to the art.
Hawkins Cheung, due to his small stature, probably has the best legs in WCK (that I have seen). He used his methods of leg checks, disruptions, counters, offbalancing on me first hand. And I sparred him and received many a leg kick with his hard dress boots on… fortunately, since I am a Chinese medicine practitioner, I have good Dit Da and acupuncture to help me forget about the pain… 
WCK needs to develop this skill - it is still a viable skill for the lower gates.