
Bodybuilding champion Bolo Yeung, who appeared with Lee in Enter the Dragon, visited Lee at his house in Kowloon in 1973, during a break in filming.
When Yeung entered Lee's office, Lee was wearing a headset plugged into a machine containing various knobs and dials. He was also sweating profusely.
Through one side of his headphones were coming loud noises - car engines and horns - while the other side produced soft, tranquil sounds like raindrops hitting a pond. The idea, Lee explained to Yeung, was to train to separate both sides so he could focus only on the soft sounds.
"He told me fighting is mainly practiced with one's attention on one person," Yeung says. "But in the street, one might be attacked by four or more people, so I want to have my senses heightened to be receptive to the slightest sound."
"Bruce also devised a particularly difficult exercise he called 'The Flag,' Yeung says. While lying on a bench, he would grasp the uprights attached to the bench with both hands and raise himself off the bench, supported only by his shoulders. Then, with his knees locked straight and his lower back raised off the bench, he'd perform leg raises. He was able to keep himself perfectly vertical in midair. He was incredible. In a hundred years, there will never be another like him."