After Choi returned to Korea in 1946 he started teaching a martial art he had learned in Japan, Daito Ryu Aikijujitsu.
His initial students and their students, etc., adapted these techniques to their own needs and added techniques from other Korean and non-Korean styles, forming Hapkido, Kuksool Won, Hwarangdo, Tukgongmoosul, Hanmudo, Hanpul, etc.,.
[1] Kim Yun Sang aka Kim Yoon-Sang (김윤상) began using the term "Hapkiyusul" in 1987, a year after Choi Yong Sul died, in order to differentiate what he had learned from Choi Yong-Sul and was teaching from mainstream Hapkido.
Hapkiyusul members practice the original techniques and learning/teaching process as taught to Kim Yun Sang by Choi.
Schools or practice groups exist in the US (Washington, Texas), Australia, Brazil, England, Germany, Italy and the Netherlands.