Kim Yun Sang aka Kim Yoon-Sang 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. Kim was authorized by Choi Yong Sul to use Choi's own name ("Yong Sul") as the name of his kwan (Yong Sul Kwan ).
Choi's students were told that all of these techniques (including the kicks and hand strikes taught by Choi) were Daito Ryu Aikijujitsu. The use of hapki (better known as aiki in Japanese traditions) is greatly emphasized.
Grand Master Choi Yong-Sool who had two successors. One was his son Choi Bok-Yeol (who unfortunately died in 1987, one year after his father) & Kim Yun Sang.