Garbage Disposal Knockout Plug

As later in the same guide it provides these pearls of wisdom.

Garbage disposal knockout plug.

The plug is similar to the knockout plug on a metal electrical box and you remove it the same way. One common use for a knockout plug is the dishwasher connection found on many garbage disposals. Lay the garbage disposal on its side with the dishwasher drain opening facing you. Each knockout plug is typically stamped from the same type of material as the tube or hole it blocks.

Your disposer is ruggedly built to give you many years of trouble free service. When knockout plug falls into disposer you may remove it or simply grind it up when the disposer is used. Move the screwdriver around the perimeter of the plug tapping all the while with the hammer so you can cleanly remove the whole knockout plug. Insert a screwdriver into the drain opening far enough to contact the plug then tap the screwdriver firmly with a hammer to knock out the plug.

Using the hammer lightly tap the back of the screwdriver against the knockout plug repeatedly.