Case length gauge explained

Screw the case length gauge (the part with the pin that goes through the case) all the way into the cutter. There should also have been a shell holder with threads on the inside, that screws onto the lock stud. This locks the case into the shell holder/lock stud assembly. 

Now, when you trim the unprimed case, the pin on the end of the case length gauge bottoms out against the lock stud when the correct case length is reached. There is a picture illustrating this on the back of the case length gauge package.