Luke
Luke
[Darth Vader is holding up what appears to be the handle of a powered-down lightsaber looking down at it while talking to Luke Skywalker, drawn with thin hair hanging down his forehead.] Darth Vader: I see you have constructed a new lightsaber. Luke Skywalker: ...Yes. Luke Skywalker: That is definitely what I did.
[Caption below the panel:] Vader finds Luke's Fleshlight.

This comic takes place in a scene from the third theatrically-released Star Wars movie, Return of the Jedi , wherein Darth Vader confronts his son, Luke Skywalker , who had recently surrendered to Imperial soldiers. In the movie Vader notes that Luke Skywalker has constructed a new lightsaber following the loss of his original during their duel on Cloud City , Luke Skywalker's original lightsaber actually having been Anakin Skywalker's second lightsaber, Anakin who later turned into Darth Vader.
In this comic, however, Darth Vader has accidentally discovered his son's fleshlight (a male sex toy designed to imitate one of various orifices, most commonly a vagina), which he apparently brought with him on the attack on the Forest Moon of Endor . From a certain point of view , a fleshlight could be mistaken for the handle of a lightsaber, without the blade extended. Like many teenagers, Luke Skywalker is attempting to hide evidence of his sexual activity from a parent. References to fleshlights are a recurring theme in xkcd.
The title text refers to the fact that if Darth Vader turned the fleshlight on, instead of creating a blade of pure plasma or energy suspended in a force containment field the device would simply vibrate, revealing it for what it really is. Randall is also punning on "being turned on" as slang for being sexually aroused.
In a later comic another version of this scene is displayed in 1433: Lightsaber . This time not so embarrassing for Luke, but much more dangerous. In that comic Luke should really have said "Don't turn it on".
In 1637: Salt Mine , Ponytail makes a very similar remark to the one that Luke makes here.
[Darth Vader is holding up what appears to be the handle of a powered-down lightsaber looking down at it while talking to Luke Skywalker, drawn with thin hair hanging down his forehead.] Darth Vader: I see you have constructed a new lightsaber. Luke Skywalker: ...Yes. Luke Skywalker: That is definitely what I did.
[Caption below the panel:] Vader finds Luke's Fleshlight.
