In the Zapruder film, Jackie Kennedy can clearly be see trying to crawl over the trunk of the vehicle to retrieve it.
Please take a look at this rendering of the Zapruder film,
http://www.youtube.com/watch?v=eqzJQE8LYrQ
starting at about 50 seconds. It appears to me that at about 53 sec, JFK is in trouble, and Jackie is even responding to something- and later, at about 57 sec., JFK obviously gets hit, but it seems to me that possibly this is not the first hit.
The Zapruder film shows that it is the right front and side that explodes outwards. It look like the wind in the open car may have blown some of the material backwards. Plus the car was passing under it as it fell.
A lot of people have said that the jerking backwards of Kennedy's head indicates a frontal shot, but this is just due to the jetting effect of the exiting brain matter.