So how dows this work?

[Tuqui-tuqui]

Take a look at this video:

http://www.wimp.com/thecart/

Can someone explain what is happening here? It's not a trick question, I'm just interested in the physics behind it

[Azazel]

This is due to the physics of gears.

Just as the larger gear causes direction reversal of the smaller spindles, so does the ruler cause direction reversal of the larger wheel.

The speed is doubled due to the larger radius of the larger gear that spins the smaller gears twice as fast.

[Minerva]

[mapes]

This is due to the physics of gears.

Just as the larger gear causes direction reversal of the smaller spindles, so does the ruler cause direction reversal of the larger wheel.

The speed is doubled due to the larger radius of the larger gear that spins the smaller gears twice as fast.

Yes don't think of the ruler as a ruler think of it as a another gear same goes for the paper too.

[MR_MADHATTER]

Right! Imagine that the table or paper were a round gear and the ruler was another gear either inside or outside that gear.
Concentric to one another. With more of the thread spools and blue wheels all the way around the inside between the concentric gears. That would be planetary reduction drive.
http://www.youtube.com/watch?v=y2vRkXoTWqc
http://www.youtube.com/watch?v=CgXCFw39Gkw
http://www.youtube.com/watch?v=VcqoLoOWOI0

[mapes]

That top one that is a triangle looks suspiciously like a Wenkle rotary engine