Your setup looks fine until it's time to set your back, to raise your chest, to send a punishingly uncomfortable wave of tension from Hell from the base of your neck to Uncle Rico's cocyx. Then things go badly off the rails. You flatten your back briefly, then you commence to yankin' the bar off the ground. Yes, actually yanking. You're not pulling the bar off the ground, you're yanking it. It's very unlikely a lifter will keep his back in solid extension when going from Zero to over 315 in a split second. And yours responds as expected: it rounds fairly significantly. You need to set your back so hard it tries to cause cramps, take a big breath and hold it, then, holding every scrap of that back extension force, you drag the bar up your shins to lockout.
Honestly, your yank and flex was profound enough that I question whether you can even do a set of 5 at 315 while holding your back in extension. Sorry to be the bearer of bad news.