A circle was cut out for a base and is now being initially sunk into a steel ring.  This shape will become a half a hemisphere so I'll need to sink it deeper than I would be able to in a bowl form.  Using a ring is a quick way to sink a shape into a deep sided form. All shapes hammered into a ring doesn't necessarily have to be symmetrically round, you can create oblong or free form configurations as well.  The reason for this is because you are hammering in the "air" and only the outer edges of the form are supported.  It's easy enough to rotate and shift your form to achieve the basic shape while being selective with your hammering.