ok lemme have a try at this philosophy:
Kosshi in kanji (as defined by hatsumi) is 'finger' and 'bone'. as far as i see, there are two main explanations to the meaning of this: the first is practical, which is using the finger or bones (in other words the tips of your body) to strike the enemy weak points. the second and more philosophical ive read somewhere is that to truly learn gyokko ryu you must train "your finger to the bone" or something like that....
Yeah... the issue with that kind of reasoning is that it only looks at a superficial (and inaccurate) level. Same with Himura's take above... looking at a single physical trait, rather than looking at what that trait is really showing you (as well as other aspects). Then it gets inaccurate when you realize that Koto Ryu, for instance, actually uses fists such as Shito Ken more than Gyokko Ryu does... in fact, Gyokko's most common striking weapons are Kiten Ken and Soku Yaku Keri, neither of which are "fingers and toes".
question is, how would you define kosshijutsu?
I'll get to that at the end...
Do you think that is the only reading of the kanji?
It's probably best to look at this a bit deeper, there's a knack to this sort of thing.
Ha, I see what you did there... punny.... (the first kanji is "kotsu", meaning bone, but can also be written meaning "a knack".... ha!)
My experience with gyokko ryu is in the kihon happo, joryaku no maki, and some muto dori. Kosshijutsu I have experimented with is performed by striking kyusho with various fists. If there is more to koshijutsu than striking kyusho would you mind helping me figure out what that is? What else do you believe I should be looking for?
So Kosshijutsu is targeting kyusho? Then is Karate Kosshijutsu? See how such a definition doesn't work? It's just too broad. In terms of what else to look for, look to the tactics and movement patterns, rather than the individual strikes/kicks etc.
So what is Kosshijutsu? Well, the biggest trait of Kosshijutsu (to my mind) is the usage of small actions to defeat larger opponents. That's where the name comes into it... the reference to "finger bones" is a reference to using smaller parts/techniques/actions to attack your opponents, with the meaning being that the bones of the fingers are the smallest (usable) ones on the body. The way that gets expressed in Gyokko Ryu is that the the primary tactic is to move around the opponent, attacking from unusual angles, rather than meeting the opponent head on. That's Kosshijutsu... the fingertips/toes against soft targets really isn't. It's a part of how the art works, but that's not the same as saying that that is Kosshijutsu, especially when it's a smaller part of the physical methods themselves.
Look at the kata. For Hicho no Kata (to begin with), the kick is "hidden" (striking from an unusual, or hidden angle) under the attacking and blocking arms.
Koku shows moving inside and outside around the opponent, deflecting the kick, and finishing with a strike from an unusual (hidden) angle.
Renyo shows constant moving around, with a hidden kick as part of the take-down, and so on.
And so it continues. Does that help?