It's been publicly known since Six Seconds in Dallas came out in the mid-60's. You can see it for yourself with publicly-available Zapruder film frames and the right software tools.
Nope, a revisited study concluded there was too much motion blur on the limo to ascertain whether the head went forward. Have another look.
That said, I wouldn't be surprised if the explosive forces inside JFK's head and a jet effect pushed it in the opposite direction of any blow outs. However, you don't get this jet effect from a FMJ bullet, only a frangible one. It looks like you can see the plasma jetting out of JFK's right temple and since his head is turned slightly to the left, you will get a vector component force pushing his head back and to the left.
Just look at the damage done by a frangible bullet.
The shot from the overpass was probably a FMJ bullet that created a small entry wound at JFK's hairline and blew out a fist-sized hole from his right occipital region. How it affected the motion of JFK's head is unknown since the explosive forces from the frangible bullet likely dominated.
If the Z film was spliced and altered it was to remove any signs of a shot from the front. That means individual frames of the back of JFK's head after frame 313 might have been touched up. It stands to reason that the autopsy photos had to remove the hole in the occipital region, as well. But those edits are easier than speeding up the limo, which involves systematically removing frames, unless you are splicing out an entire sequence of frames, such as the turn onto Elm. There are several unexplained splices on the film returned to Zapruder. So why did the FBI return an edited copy and where is the original Z film? It would tell all.
