Amazing stretching effect but how?

This site uses a really nice transition effect. very smooth change in size and shape as the different areas of the site are accessed, I’ve seen this done before on other sites so I thought i’d come here and ask if anyone knows of a tutorial or has an fla of this? (fla would be fantastic).

I’ll stand on my head in custard if i get an answer to this

Thank you.

it’s just won kirupa site of the week: