Combato had its beginning in the Liverpool theatres with jujutsu demonstrations by travelling Japanese wrestlers, Yukio Tani and Taro Miyake. As a boy, Underwood idolized these experts, and rapidly established himself as a prodigy.
Underwood was frequently requested by American and Canadian Law Enforcement Agencies to teach his Combato system during World War II
After the war had ended he refused Underwood modified the system to remove its lethal applications and instead focus on the Law Enforcement applications of self-defence, compliance and control tactics and called it "Defendo."
Between 1980 and 1981 Underwood appeared four times on The Tonight Show Starring Johnny Carson. He also appeared at that time on all of Hollywood's major talk shows including Real People, That's Incredible, and Merv Griffin. In 1980 a short documentary film about Underwood's life and work, entitled Don't Mess With Bill was nominated for an Oscar at the 53rd Academy Awards in 1981.