稽:Kei is "thinking deeply something, extremely close to the subject, gathering thoughts.
古:Ko is "old thing", "a thing coming from ancient" (for me, carrying the craft which has established old time.)
The different between this Kei-Ko and regular practice, Ren Syu is active or passive. I found beauty in this. I was born and raised in martial arts family and also has trained Japanese Classic Dance. We certainly use the term Kei-Ko. When I was kid, I hated this, because it seems to take forever to have the craft you are practicing. However, I have realized it is not just about "time" of practicing, but how you practice it. One of the books about Hijikata Tatsumi-Sensei (written by his pupil) I have read, wrote his philosophy about mental preparation for Butoh dance. Hjikata-Sensei asked his pupils "All or Nothing". He asked how you face on Butoh. You are in or out. Actively thinking or Passively receiving. If we do Kei-Ko, you will have the craft own eventually. But if we just do Ren Syu, we probably never get the heart of the craft, and it won't be yours. It requires discipline, but it worth it! Let us Kei Ko Butoh Dance!