The idea of Vader using telekinetic powers during his fight with Luke was created during story meetings. There was concern, however, that the audience might think back to the first film and wonder why Vader didn't use all his powers on Ben; but this was easily explained by the fact that Ben was probably stronger than Vader. George Lucas and Leigh Brackett also discussed the different levels of the Force; maybe Ben was a six, Vader was a four, Luke is now at level two.Source: Star Wars The Annotated Screenplays
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