This was a project in wood shop that I was never good enough to make:
It's a block of wood with a pair of perpendicular grooves cut in it, dividing the block into four quadrants of equal size and shape. Two little blocks that fit into that groove (and are held in due to the narrowing of said groove towards the top of the block) are attached to a handle that can spin freely around either of the little shuttle blocks. The effect, when you turn the crank, is for the little blocks of wood to move back and forth, never colliding, but never really doing much of anything, hence the name.
I've heard that these little toys originated in New England, but I have no idea whether or not that's true.