To go along with what the others have said, your first movement is throwing your head back to get momentum to complete the movement. Stop that, there really shouldn't be any movement in your lower back. Instead of thinking about going down, think about pushing out towards the other end of the gym, this will keep a tight arch and make sure you go all the way out until your knees are locked out and you reverse the movement.
Another thing, you're not finishing the movement at the top, you're completely short changing yourself. The point of the GHR was so you get activation of the calf, then the hamstring and finally the glutes when your at the top of the left, your glutes are squeezing hard to keep your body upright. Think about the last movement of the squat or deadlift, your hips come through and your glutes are tight as shit. You want to replicate that. Hope that helps.