Maura did an excellent job critiquing. I appreciate that you are'nt using a crest release to balance yourself. I would also like to see a tad bit more contact, though.
As for your lower leg, I am concerned about how far back it slips. You are grabbing a bit with your knee which makes contact with your lower leg difficult. This can be the "kiss of death" on XC. If your horse were to stop, you would be pitched forward and, without that lower leg forward, you would be off over the horse's ears. No fun.
Get that lower leg forward so that it stays at the girth. Let go with that knee and stretch that leg forward. It will burn a bit as you stretch, but you will lengthen the muscles on the back of your leg with work. Then, if the horse quits on you, you will have a rock solid leg to keep you from pitching forward.
Maura was talking about my release. My "modified" auto release is one I often use when riding very aggressively. It is much like what you are trying, by clamping a bit on the sides of the horse's neck. But you can see I keep contact. If the horse needs more rein, I am in a place to give it. While my lower leg seems to have slipped back, due to the steep angle of the horse, if it were much more forward, I might be behind the motion and have to be pulled over the jump by the reins. The question needs to be.....would I be able to be in balance if I were in this same position standing on the ground...or would I pitch forward or fall back. That balance is what you want. I'm not perfect here, but I am OK.
A full auto release is simply when, with contact, you have a straight line between elbow and bit. Like my avvie. I am actually hardly "clamping" on the neck here. Your lower leg needs to be under you for good balance.