The big, gently rounded, bulbous head on the Popper gives you the opportunity for heavy or gentle popping of the sphincter rings. This type of heavy stimulation to the sphincter can elicit some really intense feelings in some. Similar to the act of punch fisting, but on a much more manageable scale for most.
The Popper has a harness type base for strapping onto your favorite device. The circumference is for both the head and the second bump near the base.