This effect reverses the order in which rings are drawn. By default, rings are drawn in ascending order by size, i.e. the smallest ring is drawn first. When Convex is enabled, the smallest ring is drawn last. This makes little difference if the rings are drawn as lines, but if the rings are filled, it typically causes the resulting image to appear convex instead of concave (more like a worm and less like a tunnel).
In 3D terms, Convex is a change of perspective: it moves the observer behind the image. When the rings are seen from the opposite side, they overlap differently, which reverses the illusion of depth.
Convex is especially useful when filled rings are moved with trail. Trail causes the outer rings to lag behind the inner ones, so that the rings spread out as they move. If the image is concave, the inner rings are often hidden, which can make the trail hard to see. Convex makes the trail more obvious, by keeping the inner rings visible.