Our green belt, Ms. Joanna Bennett spotted this.
Bruce Lee practicing chi sao with his teacher, Ip Man!
Chi sao is one of the primary trapping range sensitivity drills in the Wing Chun system.
Notice Bruce starts with a bong sao & fook sao.
Ip Man starts with a fook sao & wu sao, which looks rather like a pak sao.
Like most tools in trapping range, the wu sao can easily change to taun sao, pak sao, gum sao or other techniques when needed.
That is the nature of the drill, from here each person tries to hit the other while staying in contact and simultaneously defending.