Video: Moy Tung Chi Sao - Demo of elbow position and more
Short video of Moy Yat Ving Tsun Kung Fu Chi Sao - 39 seconds of Grandmaster Moy Tung teaching on and demonstrating the role of elbows in Ving Tsun Chi Sao, as well as other techniques and details - see the centerline, form and technique control, the Ving Tsun horse, power and relaxation.
A lot of information in less than forty seconds - worth replaying a few times. While this may be eye-opening to any experienced martial artist or inexperienced member of the public, to learn and understand the Ving Tsun Kung Fu system so that you can train to develop this ability, you need to train under a qualified sifu.