Behold Jackie's mechanical and programmed behavior in the Zapruder film:
1. Jackie turns to JFK and starts looking at him (around frame 190).
2. When the throat shot happens she calmly puts her hand on JFK's elbow and moves her eyes towards Connally.
3. She keeps holding a steady gaze on Connally, waiting.
4. Connally turns his head slowly to the right and at the moment he suddenly quickly turns his head back to the left (time sync signal) Jackie equally suddenly shifts her position.
5. Jackie holds JFK's arm is a steady and fixed position and with her arms makes a clear space for a bullet path from the bottom of the backseat.
6. Jackie keeps holding the same position until the driver pushes a remote button that triggers the gun mounted in the backseat.
7. Bang! An expanding bullet (or two) shoots out from the bottom of the backseat and creates a small entry wound in a lower left position of JFK's head and a large exit wound in the upper right of the skull.
Zapruder clip:
http://www.youtube.com/watch?v=iU83R7rpXQY
Backseat with bullet hole(s):
http://i53.tinypic.com/m5w7a.jpg
Also notice the absence of stains on Jackie's dress compared to the stains on the backseat. Jackie sat in that backseat all the way to the hospital, which means that the stains have been added afterwards to obscure the bullet holes.