When does webpack generate childComplier assets?

[MrKou47]

I was reading extract-text-webpack-plugin source code recently.And i saw that the plugin start a new complier for generate css file.But i couldn't find where does the file generated.I know that assets can be generated use complication.assets[filename]=xxx, but i couldn't find the code like this from extract-text-webpack-plugin source code.So I'm confused about this part...Could you explain this part for me?