Bensen ALWAYS referred to this device as a horizontal stabilizer. The "rock guard" nickname is a bit of a joke, started by people other than Bensen. Igor claimed to prefer that the HS not be too large or powerful, lest the aircraft tend to nose down in vertical descents. IOW, lest it be pitch stable. Heaven forbid.
The Bensen HS is too far aft to protect the prop very well. Some people, recognizing this, have installed wide strips of plywood on the bottom of the keel, all the way forward to the nose wheel, hoping to deflect stones thrown up by the nose wheel.
I've never known the Bensen HS to prevent prop damage, but how do you prove that something was trying to happen but didn't?