Close up of the Dewalt ground rod bit

Here's a close up of the DeWalt ground rod driving bit.  The cup is press fitted onto the shaft, and that is why it can be separated using a hydraulic press.

The shaft has a slight taper to it.  If you set it upright on the a table, then the end closest the to the table has a larger diameter than the end at the top.  The taper is all within the cup itself, right above the cup the shaft transitions to a uniform diameter.