The family of Orville Nix is suing the federal government for losing the original of his film of the JFK assassination:
You can find a direct link to a copy of the film here:
As you may have noticed, the Nix film shows the limousine noticeably slowing down before SS agent Clint Hill climbs on. This slowing down in not so obvious in the Zapruder film, but it also was reported by many eyewitnesses.
Here is a discussion of the film and some unusual things it appears to reveal:
Yes, JFK's head snaps backward because he was shot from the front. Any other assessment is BS.