Why does a Lister’s corn hurt when I run

I recently did an episode where I was talking about Lister’s corns.

If you missed it, a "Lister's corn" is really just a callus or thick skin that looks like the pinky toenail or the little toenail has a split in it. 

That edge of the nail that looks like a completely separate piece of nail.

But it's not a nail at all, it's callus.
 
I just saw a runner who had this problem where his Lister's corn was really painful. It hurt so much he thought the nail might be ingrown. He asked,

Why does a lister’s corn hurt when I run? 
 
Well, that's what we're talking about today on the Doc On The Run Podcast.