I was taught this method also, by a Japanese woman in the 80s. I think it works because the cooker cuts the heat exactly when it's all turned to steam, which is

I was taught this method also, by a Japanese woman in the 80s. I think it works because the cooker cuts the heat exactly when it's all turned to steam, which is held at a minimum pressure by the lid. So the amount of water is very forgiving because there's those two automatic functions regulating the process.