Steve B wrote:
...

What's it called?

A "slip roller" or "slip rolls"

Here's one DIY:
http://www.metalwebnews.com/howto/rolls/rolls.html

Vince Gingery has a little how-to book for one. See Lindsy's (sp) books.

I captured this pic from a video:
http://home.comcast.net/~bobengelhardt/SlipRoll.png (700kB, sorry)
It has a different layout and uses cranks to link the 2 driving rolls,
which I like.

Bob