Peter,
Good question, it is very simple. After a cut in a particular groove, I return the rifling guide all the way forward until the grooves in the wooden guide disengage from the metal index plate. Then, I rotate the guide to the next groove and re-engage it with the index plate. I normally take only one cut per groove, then index on to the next. So, after the first seven pulls the barrel is rifled, just that the grooves need to be made much deeper. I estimate that it takes about 100 cuts per groove, or 700 pulls to finish the rifling process.
The lowest photo shows the guide and the index plate at just about the position where the guide would be disengaged and rotated. I hope that this makes sense.
Jim