If you use those 4' fiberglass alignment rods, this is a great way to make them more functional. The HEMI viewing window attaches to the top of the fiberglass rod (the end with the cap on it) and the rod can be inserted into the ground at about a 45-degree angle. Place the ball so you can see it in the window and the window will monitor any head movement.
Same functionality as the HEMI product except you can't use it indoors and you have to provide the fiberglass rods yourself. The HEMI is a great training aid with the weighted base and telescoping rod but if you simply need the sighting window then this item is for you!