Some observations from the video and thoughts:
1) Ditch the false grip and wrap your thumbs around the bar. This is how people die.
2) What part of the back are you trying to arch? Set the back from the top down -- squeeze your chest up to create the arch. Hard to tell exactly from the video but it looks like she's hyperextending her low back which doesn't accomplish much.
3) There's no need to create a huge arch if you're not a competitive powerlifter. Arch enough to slightly reduce the ROM and create tension throughout your torso.
4) Sounds like the breathing is off. Get a full breath at the top and hold it in while moving the bar.