Thank you for the explanation.They improve consistency in primer seating depth or swage depth by holding the brass down onto the shell plate so when the primer (or swage rod tip) gets pushed into the pocket, it immediately goes into the pocket.
A couple of friends are loading on the Apex-10, one motorized and one manual, and they are using them at both the swaging station (#3) and the priming station (#4) and were recommending that I get them for my manually operated Apex-10...along with the Jack Bolt. I'm currently using the factory Hold Down die over the swaging station and a Sizing die over the priming station