That was a connector sprayed down with brake cleaner to remove the build-up of oil and dirt. If yours is covered in crap it would be a good idea to clean and inspect it before you end up with a toasty sensation on your left toes.
According to some Electrical Engineering friends, for a permanent connection, both crimping and solder are the way to go! Crimping makes a better mechanical connection and solder is a better electrical connection. I took the metal crimp parts out of some crimp type butt-splice connectors and crimped those on. Then soldered and heat shrunk the whole thing in adhesive lined shrink tube. That fooker's REALLY connected now.