There's an optimum amout of jiggling that needs to be performed before rolling any die. Not enough and the result is not random enough, but too much and it's a sure way to make the dice hate you.

Transcript

R2-D2: Look, I cajole dice, I beg them, I even punish them. But never in my life have I manipulated them.
R2-D2: You know me; I always play by the rules.
{beat}
Obi-Wan: You have a point. Technically.
R2-D2: Here Sally, you take the video camera. I'm too excited to hold it steady.
Yoda: Cool!
Anakin: Okay, hand me the die.
Anakin: I leap at Obi-Wan.
R2-D2: Right, everybody back.
Anakin: What do I need to roll?
GM: At least a 13.
R2-D2: Don't jiggle it so much!
Die: DIE ARMED
R2-D2: No! Too late! Don't stop shaking it!
Anakin: What?! What do I do now?
R2-D2: Roll it! Roll it! ROLL IT!