In 1909, sake researchers made a breakthrough in simplifying the brewing process when they discovered that all the hard work involved in tirelessly mixing the yeast starter from the kimoto method (yama-oroshi, 山卸) was actually not necessary. Instead, by adding more water and keeping the moto at a higher temperature, the brewers could still produce a healthy environment for the yeast to proliferate.
The very logical name yamahai is a combination of the first part of yama-oroshi together with first part of haishi (廃止), meaning to cease. Therefore, Yamahai simply means to cease mixing by pole and this is the only difference in brewing method when compared to kimoto.
However, brewers at this point still didn't fully understand the significance of the lactic bacteria to lactic acid conversion that was clearing out unwanted yeasts and bacteria in the yeast starter. With yamahai, lactic acid is not added as it is with the method that superseded it (see Sokujo 速醸). Instead, the lactic bacteria occurs naturally, and as a result takes a few weeks to appear. This allows some wild yeasts and bacteria to enter the moto.
Eventually these unwanted elemets are eliminated when the lactic bacteria kills everything except the desired yeast. But before this process happens, the wild elements are able to impart some of the interesting flavour profiles that are the hallmark of this fantastic style of sake.