To this day there are nine professional dan levels in China and South Korean as well as in Japan; the same applies in Taiwan.
The basic system described at Go handicaps is insufficient to provide an accurate ranking, because professional levels are closer together.
In modern times that has become even tighter, since professional shodan is not generally awarded to players who will remain at that level thereafter.
This is evidence that the spreading of pro levels at three per handicap stone was a standard way of calibrating strengths.
Sen-ai-sen (先相先, senaisen) describes the handicap of taking Black (making the first move) in two games out of every three.
Theoretically, then, a professional shodan should play a 9 dan (who by definition would be Meijin) at three stones in every game.
Usually, after three or four games are won in a row by the same player (or some other agreed threshold is reached), the handicap shifts.
A player against whom the handicap moves is said to be 'beaten down', at least a requirement to acknowledge the strength of the opponent, possibly a severe professional humiliation.
The jubango series sponsored by the Yomiuri Shimbun in the twentieth century emphasised this competitive aspect, which was part of the negotiated match conditions.
The distinction between classical jubango and just any ten-game challenge match therefore lies in the drafting of the specific beating-down arrangements.