Ippon-ken: one-fist, Ippon-kowashi-no-waza: one technique to destroy, Ippon-nukite: one figher spear hand, and then finally Ippon-kumite which means to me a "one point sparring" process or technique.
To me ippon-kumite is just a level or step in the process to train a practitioner to apply a karate technique, not sport fighting, against another pre-defined technique. Both parties are restricted to either applying or countering one set of applicable techniques. This is a drill format training tool. Since ippon kumite in my system of instruction does not occur until all the fundamentals are ingrained and at least "one" kata is thoroughly covered one does not do kumite of any kind.
In my system of instruction the drills or ippon-kumite level is derived from the kata bunkai (see next for my view/definition of bunkai). This is not always true in other dojo to include other dojo that teach the same system as I do. Some actually have separate ippon-kumite, or drills, that are created from fighting techniques. These fighting techniques are those best used for tournament/sparring/competitive/sport derived applications. I believe Tatsuo Sensei as well as most of the Americans who brought it to the United States developed.
When you consider the short duration and the predominance of promotions that came from Tatsuo for their participation and achievements in "contests" which are tournament/sport oriented you see why they split from kata bunkai to a separate and unique form of instruction and practice.
Ippon-kumite is merely a tool that introduces the practitioners to fighting techniques and helps develop and train in the gross or broad strokes so proper form, stance, posture, weight transfer, power generation, etc. can be taught, trained, practiced and applied in a relatively reality based trainable fighting system.
Too many karate-ka who were exposed to the shortened versions of this tool ended up dropping the kata bunkai for sport fighting techniques and combinations that would get points quickly and achieve recognition and promotions for the trophies won, etc.
In my system of instruction I have tried to keep the kata bunkai, the foundation of techniques for the system as I see and believe it to be, tied to the actual fighting training tools that provide the foundation to applying true karate techniques to fighting, i.e. defensive applications in a combative non-regulated non-sport oriented endeavor.
Once a practitioner achieves a level of proficiency in ippon-kumite then they will move to san-bon-kumite, gohon-kumite, shiai-kumite and finally jiyu-kumite with levels and graduations of levels in and between each of these kumite/drills. The idea is not to find patterns and habits but to achieve a level of proficiency with an end result of each individual taking it beyond the drill/pattern/form level into a unique level where the person can achieve results spontaneously and according to such things as range, application of proper technique, power, and so on. Since fighting is fluid, chaotic and dangerous one must have a foundation that can be morphed and connected and molded on the fly which is extremely difficult. Ergo, why drills with combo's for sport are so quick and more easily achieved, no real effort.
Ippon-kumite at the base level is slow and deliberate enough with mutually beneficial relationships between uke-n-=tori that allows one to "see," "hear," and "feel" how it all works and connects so the brain/mind can learn and file it for retrieval when needed later, etc. Each kumite/drill adds, changes, and increases the level of difficulty. It is not set it stone either.
Each of these can be moved up in intensity as well as a more chaotic application. If you have a variety of one-step sparring techniques after learning them in order and correctly you can start to mix and match, set the drills and follow them, with a final application where tori-n-uke are free to pick and choose what they wish to apply in a one-step fashion to achieve spontaneity of a sort.
A common mistake made when practicing ippon-kumite is the tendency to remain movement in one path only, the path forward and the path backward. It must be practiced this way as a basic, an introduction to ippon-kumite, yet to achieve proficiency one must remember that there are many directions. In the ken-po goku-i it is taught that the body should be able to change direction at any time. This is true and we have many directions with only eight primary to start with and lets not forget that we also must move out of a more linear fashion and include circular movement.
This model goes to the other forms of kumite, i.e. sanbon-kumite, etc. Pay attention to your clear and exclusion zones as well. What I mean by this is that many tend to start ippon-kumite in a set stance, at a range that is within attack distance, i.e. exclusion zone, but to truly use this tool to its fullest you should always begin ippon-kumite outside of range then as the uke moves into range you as tori "move." That movement starts forward or backward yet as you progress it must take on a direction related to the techniques being practiced.
Lastly, as a reminder, all this cannot occur correctly nor be adequately assimilated into the instinctual part of the mind unless the practitioner has thoroughly learned and achieved proficiency in the fundamentals for it is the fundamentals that support the system.