What does "neutral gas" mean and why?

Zhongqi has two meanings. I don't know which one you mean.

To put it simply, Zhongqi is a solar term in an even position among the 24 solar terms, and every other solar term in chronological order is a Zhongqi. Strictly speaking, this is not the lunar calendar, but the solar calendar divides a year into twelve solar terms and twelve neutral qi, among which the neutral qi is: rain, vernal equinox, Grain Rain, Xiaoman, summer solstice, summer heat, summer heat, autumn equinox, first frost, light snow, winter solstice and severe cold.

There is another saying: the middle qi refers to the middle jiao qi, because the spleen and stomach are located in the middle jiao, that is, the spleen and stomach qi. Traditional Chinese medicine calls the spleen and stomach the foundation of the day after tomorrow, so it can also be said that it is the essence of Shui Gu, and in modern words, it is the basic material for supplying and maintaining human life.

Therefore, the treatment of deficiency of middle qi is to strengthen the spleen and stomach, so as to strengthen the spleen and stomach. Traditional prescriptions include Buzhong Yiqi Decoction, Shengzi Decoction and Xiangsha Liujunzi Decoction.