Captive Plug Inside Shower Drain
 

I popped off the drain cover from my shower floor and found a large
rubbery plug inside. It is installed in such a way that its diameter is
bigger than the metal shaft above it. There is a string on it. I
realize that the plumbers in Hawaii do things bass-ackward, but has
anyone ever seen anything like this? How do you remove it to get a
snake down the drain pipe? It seems that the section of pipe that
secures the drain cover is set firmly in the grout, so I am wary of
trying to rotate it out (or worse, introducing a leak).