The rule had to do with lightsaber battles. Hamill said in the interview that he’d been working on a fight sequence with stunt coordinator Peter Diamond and Darth Vader’s double Bob Anderson (who was an “Olympic fencing champion”), and he called the training “intensive.” He explained what happened, saying:
“We had worked out a sequence we were all particularly proud of. And I’m talking weeks and weeks of this … and we said, ‘Let’s bring George [Lucas] in.’ We brought George in to look at what we had done and he said, ‘Um well … You can’t take your hands off a lightsaber. You can’t hold it in one hand.’ And we said, ‘What?’ We had choreographed stuff where, you know, we did spins around and we did various things … He didn’t want us to ever take both hands off the hilt.”
They had to restructure the fight, though Hamill said Diamond and Anderson were really helpful. “It was frustrating, but I was very lucky to have Peter, who was so skilled as a stunt coordinator, and Bob. He was someone who, he could counter if I made a mistake, he could counter it and incorporate it into the routine, and we could keep going. It takes an expert to make a novice look good, and that was certainly the case with him.”
In a 2014 “Star Wars” lightsaber featurette, Hamill gave another reason for the two-handed stance. “George was adamant that these things were really, really heavy and that we couldn’t take a hand off. We always had to have two, like Excalibur, it’s like forty or fifty pounds of weight.”
Lucas added that they wanted to speed up the fights as Luke improved his technique. “And so we very slowly started moving away from the two-handed form … to sometimes using a one-handed form.”