He inverted the faces of the part using its CFrame.
Spawn a part.
Type this into the command bar:
cf = workspace.Part.CFrame
x,y,z,r1,r2,r3,r4,r5,r6,r7,r8,r9 = cf:components()
workspace.Part.CFrame = CFrame.new(x, y, z, -r1, r2, r3, r4, r5, r6,-r7, r8, r9)
Play around with this, why it happens, I'm not sure but it's somehow explained here:
https://scriptinghelpers.org/blog/the-power-of-cframes |