w = script.Parent -- the weld
torso = w.Parent
w.Part0 = torso
w.Part1 = torso.Parent.Arm1
w.C0 = CFrame.new()--from the wiki
w.C1 = w.Part1.CFrame:toObjectSpace(w.Part0.CFrame)--from the wiki
The above script works the exact same as without the stuff from the wiki. It puts Arm1 inside of torso, when it's supposed to keep Arm1 in the same rotation and position in relation to torso. So, either the wiki isn't being helpful, or I interpreted the wiki incorrectly.

2 comments
I literally posted a weld function that you can use.. Refer to https://scriptinghelpers.org/questions/29292/welding-to-a-player
It's not for welding to the player.