I've seen similar examples offered up before, and frankly I think they're a bit weak. There is not much similarity between a pumpkin and a human skull. A pumpkin's interior is mostly air**, has nothing analogous to bone, and is not connected to anything else. Using the pumpkin model, we should expect a solid blow from a baseball bat to send one's head flying. There are better explanations for Kennedy's motion after the head shot which don't involve a shooter from the front.
** OMG!! The WTC towers were ALSO mostly air... obviously JFK was killed by means of controlled demolition. WHAT DID JFK KNOW ABOUT THE 9/11 PLOT!?!?!?!?!?!?!?!?!
What JFK knew about 9/11 is addressed in the presidents doodle book.
Source.
