When Vader is being fitted with the helmet and subsequently breaks free of the shackles, George Lucas decided at the last minute to change the position of Vader's arms from up to down by his side (the original shot can be seen in the trailers). This is why, after breaking free from the bonds, Vader appears to raise his arms, when in fact it is the necessary transition from computer-generated arms to live-action arms.
Star Wars: Episode III - Revenge of the Sith
The evil Darth Sidious enacts his final plan for unlimited power -- and the heroic Jedi Anakin Skywalker must choose a side.
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