Forster uses a mandrel to size the neck also, the tool itself has a pilot sized to the neck. You're supposed to run the mandrel in the case which makes it the right diameter and perfectly round. After using the mandrel you neck turn.
If I'm going to neck turn new brass I do it first thing. I run the mandrel in the new case, neck turn, then chamfer and size the case. It has to be resized after neck turning to return the neck to the proper dimensions.
