The term 'koshi' is Japanese and basically translates to the connecting area of the upper and lower body; hips; waist; the area full width of the lower back.

The term 'gamaku' is an Okinawan term referring to the connecting muscles of the torso to the pelvis which dictate the speed/power allowing control of the coordination between the upper and lower body.

sounds like the same thing right? right, thats why many use the term koshi/gamaku interchangably. but when thought about in terms for our use (mechanics principals), the distinction becomes less subtle.

In an interesting article/interview :
among lots of other topics of discussion which could be generated from this, I came across this quote:

William Haff: So you use bo training to help you understand karate?

Toshihiro Oshiro: Yes, but this is common sense for any serious martial artist. I study to deepen my art, both weapons and karate. The old teachers used to do that too, I think. They would watch other styles or talk and practice with other instructors to add some new technique or maybe to just check their own practice But by comparing the two arts, it is possible to see how karate used to be or is supposed to bethere is a lot of karate lost in history, and I am very interested in that. Early in my karate training Nagamine Sensei talked about the difference between koshi and gamaku, your sides vs. your lower back, in making power and focus. It wasn't until I had studied bo deeply that I found what he was talking about.

wow...three topics of discussion in one quote, I thought. (1. weapons training to improve your empty-hand technique. 2.The 'old days' of cross-training. 3. sides vs. lower back, in making power and focus.)

but this thread is only about #3 ....

I believe he was making the distinction as a point of teaching to let your muscles lead your hips. use your side muscles to drive your hip...not your back muscles. The added weight and dynamics of holding a bo would illustrate/exagurate this principal nicely I would think.

All of this came about when I was looking for an answer as to why Matsubayashi has a slight lean forward in technique. At least now I have a theory...perhaps Nagamine was stressing the use of gamaku. by leaning slightly forward and nearly aligning your back angle with your rear leg angle, it forces a student to rotate hips with side muscles. perhaps later in training, the lean forward becomes less pronounced yet the principal remains intact with the experienced student.

The debate: what do you suppose Nagamine was talking about as it relates to koshi vs gamaku?