When the setting is enabled it will automatically set the text direction for cooked post content, topic titles, category names, and the composer’s preview area.
The text in the preview window is set to the direction of the language that is input. The text direction of the composer’s textarea input defaults to the direction of the user’s preferred locale. At the moment, it is only possible to have a single text direction for the composer’s textarea input. To make it easier to add text in the opposite direction, there is a Toggle Direction button. It doesn’t affect the markup, it just makes it easier to type:
I will write a detailed topic about using the setting.