I have already read the related posts.
www.limesurvey.org/forum/can-i-do-this-w...re-entering-comments

But I still don't know how to use it.

I also wasn't able to find documentation how to use this feature.

Here : manual.limesurvey.org/Question_type_-_Mu...ommented_checkbox.29

This is what I understand.

What I don't understand is 'Remove text or uncheck checkbox automatically'?

Ah,

You're right : it's (normally) to activate the javascript solution (or not).
-> If it's at NO : when user click on checkbox : we don't touch the text input
->If it's at yes and you set "Checkbox is checked" : when user uncheck a checkbox : text input is empty.