It's a smaller, stick-mounted version of the thing in post 13 above. When the lines on the two levels appear to be one unbroken line, then (theoretically) your "vision center" is where it needs to be (directly over the stick) in order to see most accurately where your stick is pointed.
pj
chgo
Which to me seems to be pretty useless, as it is at the joint and not visible when you are down on a shot. It boils down to an alignment aid for standing behind the shot. The starter instructions that the manufacturer/inventor provide confirms this as the intent behind it. Video on this link.
http://www.rileyleisure.ca/riley-sightright-UK-CA-NZ-AU
The reason I say that I feel it is useless is, that once you finally are able to "see the line", you still have to get your body out of the way, and bend into your shooting stance, all the while maintaining the cue in perfect alignment. Once you are down on the shot, the SightRight is no longer in play. You cannot use it to double check your alignment, as it is out of your field of vision.
On an interesting note, they also make a $316.97 version that is a full 30" cue butt.
http://www.rileyleisure.ca/products/riley-sightright-extension
On their own sales ad, they say
30" Extension With Integrated SightRight Technology Included To Enhance Rest Play
So..... for almost 10 times the cost of the mid joint extension, you can have a shiny red cue butt, with an empty spirit level vial containing "SightRight technology" inlaid near the joint to help when using the bridge.
