Posted By David Brodeur
As for "everything else" being invisible, what else did you try it on? The filter works by blocking all light except maybe a 10nm band. The OIII emissions are passed virtually unattenuated, making certain objects (planetary nebulae primarily) more obvious. Note that it won't make them any brighter; it just dims the surrounding stars and any hydrogen emissions, so the planetary stands out.
One use for these is to "blink" planetary nebulae. Put the filter in front of your eye while looking at a low-power field containing a small planetary (e.g., the Saturn nebula in Aquarius, NGC 7009). All the "stars" but one will dim; the one being the PN of course.