Cyclo Chain Wear Indicator
I would say is that it is a very easy tool to use. The size is great. I have a Topeak chain tool also, but that is tiny and quite fiddly to use, especially if I'm wearing gloves.
As ever it is another well thought out Weldtite/Cyclo tool offering the home mechanic a cost effective alternative to the big boys in the form of an easy to use and manage piece.