You jerk your head up, and jerk against the bar as part of (attempting) to put your back into extension, but never really squeeze up tight. I see this quite often. Squeeze up harder at the beginning of the lift. It's not a jerky motion - it's a wave of extension that travels down the back.
Also, you need more knee extension at the beginning, and you need to stop overextending your lumbar spine at the top of the rep. The latter is a mistake with potentially hazardous consequences. Just stand up straight, and lift the chest.