Here is what JKA Chief Instructor NAKAYAMA Masatoshi says about one step sparring:
*
The people of my generation were required to study martial arts beginning in grammar school, and continuing all the way through graduation from high school. Karate was not taught in the schools at that time, so all of us had studied judo or kendo. I began kendo training in grammar school, for example, and my friends had also practiced for a long time. But judo and kendo were centered around combat -- throwing an opponent or actually striking an opponent with a sword. So the idea of combat was deeply ingrained in us, and we really needed the combative aspect which karate lacked.
Master Funakoshi understood this, and he began to change his teaching methods to meet the needs of our younger generation. We needed more than just kata all the time, and he realized that things would have to if he was going to attract young people and see his art grow.
So, he picked techniques from the kata and began teaching gohon kumite (5-step sparring) based on individual techniques. We would step in 5 times with the same attack while the defender blocked. Then the defender would counter-attack. But we had high spirits, and if the defender did not counter-attack immediately, we would attack him again, and he would be forced to improvise a defense and try to counter again. These actions became the basis for free-sparring. It was just a natural outgrowth of spirited young people practicing with one another.
Shortly thereafter, we began kihon-ippon kumite, or 1 step sparring. In this method, the attacker would announce the target area to be attacked, face or stomach, and would then execute his strongest, most powerful technique. The defender had only one chance to make a powerful, correct block and counter-attack. This was very much in keeping with the basic philosophy of martial art which revolves around the concept that there is no second chance. Everything must be done correctly the first time, or the person dies. We weren't trying to kill each other, of course, but we were trying to execute that one, perfect technique which would stop the opponent in a real fighting situation.
A natural outgrowth of this kind of training was jyu-ippon kumite (one step sparring) in which the defender knew the area to be attacked, but in which the attacker could maneuver freely for position and distancing. The significant thing about this is that this was the first time karate had been taught in any way except for application of kata movements to self defense, and the entire system of kumite (sparring) developed in a single, 5 year period. When Master Funakoshi published Karate Kyohan (The Master Text of Karate) in 1936, he included basic sparring methods in the book, and this was the first time this brand new idea was introduced to the public at large.
.
I began training in 1932, and basic kumite was introduced in 1933. In 1934, jyu-ippon kumite was introduced, and jyu kumite (free sparring) began in 1935. In November of 1936, we formed the All Japan Collegiate Karate Union and gave a demonstration at the Tokyo Civic Center. For the first time in history, we showed the public the new training methods of kumite, and demonstrated how the student progresses from 5 step sparring to 1 step, then to semi-free and finally free sparring.
*
In other words, one step sparring was an intermediate step to teaching free sparring, or karate style sport sparring; it was not a vehicle to teach self defense.