I had to think about what that meant for a moment, as it's sort of part of the weirdness of the bottle. Really, putting a cork in it is probably closer to putting a lid on a bowl, no?

It's just sealing the local minimum. (Maximum? I don't really know what the sign should be for this. Probably maximum, for enclosed volume, since I'm not sure if it's reasonable to talk about the topology having a minimum in this context...)