It's just a widget that goes on the hammer with a set screw to hold it there. It sticks out to one or both sides of the hammer, allowing you to control hammer movement with your thumb off to one side. If you scope your lever rifle, especially if the scope is low mounted, the hammer extension will let you cock or lower the hammer without pinching your thumb between the hammer and the scope. I've never used one on a lever gun, because I don't scope mine, but I did use one on my T/C Encore.
