Winding, or winking, sticks are used to determine if a board has any twist along its length. By placing one stick at each end of a board, parallel to each other, and then sighting across the tops of them, any twist will immediately become apparent. One common design has a contrasting wood inlaid into the top of one of the sticks, which makes it easier to see how level they are to each other. This design, which I saw here, relies instead on the two half moon cutouts. When sighting down the sticks, the light seen under the piece of wood across the top of the half moons becomes the contrasting visual reference.
The strip of wood above the cutouts is also beveled, which makes a nice shadow line to enhance the difference in color. I was a bit skeptical at first as to how well they would work, but after making a pair, I am sold on the idea. The one thing to consider is the background: If it is a dark color, it is harder to see light through the cutouts. With a light background, they preform beautifully.
These sticks are made of Black Walnut, quarter sawn, and come in two sizes: 14″ or 22″ long, by 2″ tall, . and are tapered from bottom to top. They are finished with two coats of Minwax Antique Oil, which gives a nice dark non-glaring finish. Sold as a pair.