Disk 1 with slot b can turn about fixed axis B; disk 2 with slots a can turn about fixed axis A. In rotation disk 1 , whose slot b is slightly wider than disk 2, periodically locks disk 2 against rotation. In rotation, disk 2 whose slots a are slightly wider than disk 1, locks disk 1 against rotation, so that simultaneous rotation of the disks is impossible.
$334$SL,SD$