What is ruling related to 'Forefinger rule' of prayers?

There are many hadiths ruling on the 'forefinger rule' but I have no problem with them. I have studied the book, 'Guide to prayer from beginning to end'. There I read hadiths and also from other sources, I got knowledge about the 'forefinger rule' but I didn't get any of them ruling on raising down the finger along with raising it up. So my question is: Should we lower our index finger after testifying with it or is it to be still raised until end of prayer? And also tell me when exactly to raise it and, if it *is* to be moved down, when it is to be done exactly?