How can I lock the shaft of a weedeater FL25 gas trimmer?

I have a featherlite FL25 weedeater and I want to change the line head for a pivot trim two head. I have taken the main head off and when I try to thread the nut that starts on the shaft and then it stops. I am told that no tools are required, b ut I don't know what locks the shaft in order to make the nut go all the way to the top of shaft. What is used to lock shaft?