Sometimes things are just taken for granted with a character. Most people would probably assume a character is right-handed unless you explicitly say otherwise. Or that a wizard can read, or that a barbarian can't. Or that someone is human unless some other thing is specified. Or a myriad other things.
Sometimes this is fine and the assumptions just work. But they can also be used for sneaky surprises.
Whether such surprises are good or not depends what side of them you're on.
Commentary by memnarch (who has not seen the movie)
Wow. This scene definitely looks like something that'd use the grappling rules. I'm still not quite sure how Rey managed to get out of having the guard's weapon pressing down on her without getting injured. I really want to see a slow motion clip of this now, just to watch how it all happens. Sure, I could end up being disappointed if it turns out the guard really was surprised by the sword drop and backed off just enough to let Rey out, but that's just part of finding things out for sure. Sometimes you don't end up with what you hope for.
Also, referencing an older campaign? Amazing. But then, Pete ought to be. He's been at this for like 20 years now! I guess he did like that part of the campaign, even if his character ended up staying dead and he left for a while.
Transcript
GM: Ooo-kay. You drop your sword. But dropped weapons Tumble Uncontrollably; that’s a self-inflicted laser sword damage table roll.
Rey: The plasma blade has negligible mass. I was holding the hilt at the centre of gravity. There’s no torque, so it drops without rotating.
GM: Well, maybe, but—
Rey: 17! I Wrench Free and drop to my knee.
GM: As I was saying, your right hand was Immobilised in a High position. Moving it down takes a full segment. The sword will hit the floor first, then Bounce Uncontrollably.
Rey: I admit it: You are better at grappling rules than I am.
GM: Then why are you smiling?
Rey: Because I know something you don’t know.
GM: And what is that?
[SFX]: catch! {Rey grabs the falling sword in her left hand}
[SFX]: whoooom! {she swings it...}
Rey: I never specified I was right-handed!
[SFX]: Sliiice! {... and strikes the guard across the knees}
[SFX]: Backhand decapitate! {on the return left-handed backhand Rey slices the guard’s neck}
Guard: Inconceivable!