Your ass shoots up before the bar moves, so you gotta keep your back tighter (like previously mentioned)
Start pos on the 225 vid looked good, the heavier ones start position looked alright, but the instant you think of starting the pull, your back completely loses its arch.
Foot placememnt under the bar can be messed with, sure, just gotta start pulling back and not up. Imagine an atlas stone on your shoulders, that you want to roll down your back. When you lean back with the bar and your chest goes up, you'll "rock" back into position, and lock it out.