This is one of the first MAC sharpener designs.
This coarse grit stone should be soaked in water for 2 minutes before use.
Use it like a file and slowly work your way up and down each side of the blade. The "foot" near the handle helps to gauge the proper angle.
Advantages: Inexpensive. Works on almost any straight-edged knife (including other brands). Coarse grit quickly resharpens knives that were extremely dull.
Disadvantages: Requires proper technique to use effectively. Wears out after lightly sharpening 50-100 knives or heavily sharpening 10-20 knives.