Variable ND/CP filters

Can someone explain how a filter can be a Polarizer and a variable ND at the same time? I suspect that there are in fact two CP filters with one fixed and the other rotating. See this. Also what does “no X spot” mean?

You’re right. I haven’t kept up with them but they first came out they had a significant X pattern of light/dark areas radiating from the center, depending on how far you rotated the front element and how much you increased contrast in the image. If that has been largely solved, they are a handy tool.

Thanks @Diane_Miller