定先 (josen) is the Japanese go term describing the phase in a series of matches between two players - like a jubango, where one player takes Black all the time. This is equivalent to a difference of one stone, or a "one-stone-handicap game" .
If one player leads by four games (six in older times), the handicap shifts one notch more advantageous to the loser. In confrontations between top players, players were spared the embarrassment, for the series would have been suspended long before even the josen phase. A game which if lost would result in a shift in the handicap is called a kadoban.
Between amateur players handicaps usually change one stone at a time. Between professionals the change was much smaller, see jubango handicaps for more information.
 Even though taking Black with no komi (and hence, josen) sometimes is referred to as "one-stone-handicap", it actually gives Black just half a move's advantage. If Black passes the first "entire" move, that doesn't balance the game but results in a likewise advantage for White.