How do I rotate Welds in a loop?

So, i'm kinda new to Welds. and I want to make a script that Rotate a fake arm. So when a Player presses a key, it Rotates the Player's Character's Arm up and then goes back down. So can someone explain how rotating and moving welds works? Thanks

The Roblox Character is connected by a bunch of welds, which keep together the limbs to the torso. The weld for the left arm would be found as such

Player = game.Players.LocalPlayer
Char = Player:FindFirstChild('Character')
T = Char:FindFirstChild('Torso')
RS = T['Right Shoulder']
LS = T['Left Shoulder']

for i = 1,20 do
  	RS.C0 = RS.C0*CFrame.Angles(0,0,math.rad(9/2))
  	LS.C0 = LS.C0*CFrame.Angles(0,0,-math.rad(9/2))
  	wait()
end

That block of code would rotate both arms forward 90 degrees. This works in a similar fashion for both legs.