What were you watching? Zapruder was near the Grassy Knoll, but not on it. If you are talking about the “spray” of the material from inside JFK’s head, it DOES NOT fly forward. It flies toward the left rear of the limo.....away from the Grassy Knoll. In fact, the motorcycle cop (Hargis?) riding behind that left rear corner of the limo was “sprayed” with blood and brain matter. He and his bike were covered......
Dude, the cloud of ejected tissue material doesn't fly along with the car hovering over the President. The car is in motion moving forward, the motorcycle cop behind it is in motion moving forward. He will ride into the cloud of material immediately afterwards.