[null and void]

Back in the day, motor oil came in cans, and one had a separate spout that punched through the top of the can. When inverted over the filler pile the oil would glug out of the can, and the bottom would pop in and out as the oil flowed out and the air burped in to replace it.

That is sorta like what I think happens in the center of a plate that is being squeezed, the center can pop up and down a bit with the changing forces (Atlantic sea-floor spread vs mountain building for example).

But as your tagline says...

[Pylon]

Oil came in cans? Next thing I know, you are gonna try to tell me that soda came in glass bottles!