Donnie Yen Chi Sao with Grandmaster Ip Chun: this is a lot harder than it looks. You have to stay in contact with the opponent's arms at all times, the idea being you simultaneously block his attacks and try to keep him off balance.
Wing Chun is an art in which principles are more important than specific techniques. This does not mean that Wing Chun does not have techniques. It means that at the root of all techniques are underlying principles.