Shower Drain Knock Out

[bdavid50]

I am finishing a remodel of my basement bathroom. I had new plumbing drain system installed. The plumber installed an Oatey drain hub that has a knock out (down about 3"). I am concerned that when I try to knock it out it might fall into the drain pipe. Is there a ledge to keep the knockout from falling down the drain?
I know this is a simple question, and even though I am relatively knowledgeable I have no experience in this area.