Originally Posted by
spar
It might help to think about starting the rep by pushing the ground down instead of pulling the bar up.
It does that because you allow it to do so.
You keep it from doing that by not doing that.
Is there a weight, even a stupid light one, at which you can do the movement without losing extension?
If so, take it back to that weight and do not allow yourself to lose back extension, ever, as you work your way back up. If you fail the lift, you fail with your back in good extension. Don't permit yourself to do any ugly reps.