In Return of the Jedi, what did Darth Vader adjust on Luke’s new lightsaber when he says 'I see you have constructed a new lightsaber'?
Answered by: Arthur Huan,watched Star Wars Episodes I~VI many times since childhood
After Vader finishes saying “I see you have constructed a new lightsaber”, the shot cuts to this:
His right hand is holding the lightsaber, and his left hand (which is on the right side of the photo) is resting on the lower part of the lightsaber.
After that, Vader moved his left hand further down the hilt, revealing to us the square plate, which is the lightsaber switch. (It makes sense, his hand was just resting on the switch to switch the lightsaber on.)
Then he turned the lightsaber:
And then turned it the other way:
After which, he said, “Your skills are complete.” and switched off Luke’s lightsaber.
That was what happened onscreen.
Vader did not adjust anything on Luke’s lightsaber, he was merely turning it around to inspect it. It’s easier to tell if you watch the original scene.
Vader wasn’t meddling with Luke’s lightsaber in preparation for a potential duel. He’s Vader. His skill is way superior and he knows he will dominate Luke if it comes to that. There’s no reason for him to meddle with Luke’s lightsaber.
Rather, Vader was inspecting Luke’s lightsaber and looking at its quality.