My impression is that if you have good posture to begin with, you might just "lift right" off the bat. I have to do a ton of supplementary work to balance out my posture issues. And the progress is slower than people generally like to admit.
But if you are low-bar squatting, your back isn't going to be completely upright, and so keeping a significant weight at a angle requires muscular contraction. Your back is going to get worked.