What Glenn said.
Also, don't throw your head back on that snatch - drive through the floor with your legs instead. WOrry less about extending your hips and more about extending your knees.
Try doing a vertical jump without opening your hips - not too good. If you really focus on driving down through the ground with your legs, while staying back over your heels more, you'll extend your hips just fine - PLUS you'll actually be generating vertical force rather than pushing the bar (and yourself) forward).