Shape your own seat

what is this sorcery?

I have seen like 10 versions of this and still have no fucking clue as to what's happening..

I gotta call bullshit

I'm guessing it's possibly some sort of Memory Foam like the mattresses that stretches instead of moulding your body shape. Because when she sits down it stays firm.. Yeah I'm going with Stretchy Memory-Foam.

Needs a buy button.

can somebody explain what's happening here?

It's a cardboard or similar material glued in honeycomb pattern.
Could't find better example but I think that you can get the basic principle how it's done by looking at this honeycomb Pom-Pom tutorial.
http://blog.mrprintables.com/how-to-make-honeycomb-pom-poms/

If it's dense enough to sit on when stretched out, just how dense is it when squished?

Its what @suko says!

This is creepy.

What sorcery is this?

Looks like a giant slinky.