I watched the first 15 seconds of the video. Beyond what Dave has written above, I'd like to see you start with your hips under the bar.
right now, you're set up leanign forward, with the bar over the front of your feet. Not a good position. You need to bring those hips in line with the bar which should be in line with your feet.
Your first move is ass back, like you're doing.
It might help with your spinal lardosis - arch - if you start out that way with your chest out as Dave has recommended.
Keep up the Good work.
All the best,