Welding involved the mixing of various elements to create something new and unpredictable in play. Often players would 'cheat' by reading the Monster Manual and DM Guide on the sly and know the proper way to quickly dispatch it.
With welding you never quite knew what to expect. A seemingly harmless bunny rabbit might be a blood thirsty killing machine. A magic item may be a fake or made from something poisonous.
The first time I remember seeing it in action was at a session where the DM had acquired Monster Manual II.
We had all loved the Fiend Folio when it came out (seems that's not a view held by many, but all three DMs concurred that if they could only have one Monster book they would choose FF every time), but we were all a bit skeptical at the collection of monsters in the new volume. Finally one went out on a limb and purchased it. It was underwhelming to say the least. When asked about it a week later he was kind, but obviously disappointed, in his review of the tome.
Not being very impressed with many of the creatures he started blending features and making creatures that people couldn't identify right away. Soon we were all doing it.
From then on your paranoid ass better be armed and ready. Who knew if that cute little squirrel sitting on the tree limb was real or just an illusion cast by a mad man who lived in the forest and didn't desire any company. Usually when the squirrel breathed fire you knew it was on.