Just watching Greg do Chia Sao is a lesson in itself. Crisp, clean, automatic. Controlling the center. Making an angle. Actions from the elbow. Facing.
Loi Lau Hoi Sung, Lat Sau Jik Chung. Receive what comes, follow what goes, upon loss of contact, attack without hesitation. As David Peterson says, this is Wing Chun in a nutshell.