Using SetPrimaryPartCFrame on a welded model?

I have a model that is welded to the player's torso when unequipped, however I want to have it welded to the player's hand when a tool is equipped. So I looked on the wiki and found something called SetPrimaryPartCFrame, so I decided to use that to see if I could position it to the player's arm.

sword:SetPrimaryPartCFrame(CFrame.new(char:FindFirstChild(plrLimbs[3]).Position))

It works, but only when I don't weld the model to the player's torso, otherwise it just stays on their torso.

Should I be using SetPrimaryPartCFrame, or am I wrong entirely? Would trying to manipulate the welds themselves work, if so how would I go about doing it? Better yet, is there a workaround that would let me break the welds on the model to their torso so I can weld it to the player's hand, then do the reverse when the tool is unequipped?