This option becomes active the Bounding Curves option is checked. Areas restricted by a bounding curve, cut off the operation at the bounding curves. This "opens" the cutting passes and provides two possibilities for cutting, Zig-Zag, or Zig.
Zig Zag
The tool moves back and forth in opposite directions as it moves from one contour depth to the next.
Zig
The tool lifts up at the end of every contour cut, so the cuts at each level are always in the same direction.