Pull My Finger Meaning

Example: A phrase used when playing a prank regarding flatulence, in which someone is asked to pull the finger of the person playing the prank, who simultaneously flatulates so as to suggest a causal relationship between the pulling of the finger and the ensuing expulsion of gas.
  Bob: "Oh, how do you play this game?"
  Charlie: "Well, Bob, just pull my finger."
2006, Marsha Collier, Santa Shops on EBay, page 89:
  Could there be a more romantic gift than a remote-controlled fart machine or a pull-my-finger pen?