Cold connections! Rivets, gears, etc. Permanent connections without soldering - riveting is simply making a hole, putting a pin into it, and flaring the ends to stop the pin from falling out! Generally rivets come with one end already flared out - you just drop in the hole and flare the other end. Rivets can be solid - like tiny blunt nails, or holow tubes - which make nice little rings with a hole when flared - or semi tubular - hollow with a head on one end. Excellent for the Steampunk esthetic, or just where ever you can't or don't want to solder, but need, or want, a mechanical connection.
And this brilliant little video from the manufacturer explains it all!