You should watch the Orville Nix film. How does a man jump off one car that is behind and then catch up to another car that is in front and then climb aboard it? There's only one way: if the cars had nearly or totally stopped. And that's what we see in the Nix film. But why don't we see it in the Zapruder film? Because they messed with it, that's why. They removed frames to hide the stopping or near-stopping of the limo.