A rattleback that will happily spin one way but not the other. It will spin clockwise with no problem but will resist and even change direction if spun counterclockwise.

The function is based on the internal weight distribution of the piece, so you will need 100% infill. Also recommended to slice with 0.05 mm layer height to make the surface is as smooth as possible on the underside, it will improve the function.

Also please note the comments that people often find it works better at 125% size.