You might be interested in checking out this site. The backward movement of Kennedy's head is dealt with in the "Dealy Plaza" section
Interesting reading.
If anyone has seen the Zapruder film at real speed you can clearly see that the Secret Serviceman driving Kennedy's car noticably slowed the vehicle just before the head shot [this is no hype, check it out yourself]...once the head shot struck home the car speeds up considerably and away.
Until someone can explain the practical motive behind slowing down a vehicle in the middle of an assasination attempt [I "KNOW" that they are taught to maintain speed at all times, especially when in a small convoy, with vehicles behind...and once anything goes down..the order is to Haul A** and never "slow down"] I will always think something was definately afoot that day in Dealy Plaza.
Seriously...I want to hear a reasonable explanation for this. I am ready to accept the truth. I am no "buff" or minor league theorist...I am simply going off of what I personally saw.
That Limousine almost comes to an abrupt stop...