So , why is it used for or where, for what purpose?

Что? День?
You tell me... What I can find is only some silly book with such name.

Exactly man that’s it!!!! It is a book about Russian spetsnaz... which is I am reading it now but unfortunately my Russian is not very good yet so I did not get it why such a sentence is used in such a book ? May be it has a special or peculiar meaning for a unique field or sth!!!

I have no idea. It can be just a fancy name or "курок" can be a nickname of some personage

Ok man thanks ...