The hydraulic catapult was a piston mounted crosswise (athartships) and aft of the start of the catapult track. It pulled on a long cable, which was turned by pulleys, first to go from sideways to forward and up, and then at the front of the track to go back to the launch point, where it was connected to the catapult shuttle, which was the shoe that extended above the deck to which the bridle was attached.
As a result, when the catapult was "fired", no matter that the shuttle had been tensioned against the holdback, there was some elasticity in the cable/pulley system such that its initial acceleration was even more sudden than the hydraulic piston's initial movement, what engineers would call a snatch load.
To compound this effect, the F4D's bridle pulled the nose down at the onset of the snatch since it was acting along a line quite a bit ahead of the aircraft's center of gravity. The nose gear shock strut was first compressed so it almost bottomed, and then as the snatch load fell off, rebounded to cause a noseup pitch. This happened so fast that there was enough time for at least one more violent oscillation before the Skyray reached the end of the catapult stroke.
The effect on the aviator was disconcerting to say the least, particularly at night. Imagine getting hit in the head with a baseball bat, twice, and then having to regain your wits and start flying, on instruments. However, based on the very low incidence of launch accidents, it wasn't so bad that they couldn't cope. Nevertheless, the steam catapult, with its more gradual onset of motion, was a welcome relief.