I have presumed that the work of keeping your lower back as straight as possible is where the lower back muscles get sore. When you get too fatigued or "let your guard down" with a heavier weight it can leave you with pain in the lower back. Obviously there are 2 kinds of pain- I prefer the pleasant soreness that proves you've worked!
From what I understand the waist should stay straight but you do bend forward a bit at the hip joint only. Is that what you mean, Jennifer? At least that's what I notice when I see Cathe saying, it's OK to lean forward just slightly during squats, or something to that effect. In other words, the spine should stay absolutely straight, and all the while your effort goes into keeping it plumb (perpendicular to the floor) it *will* slant a little forward, but still be a straight line. Your eyes should cast forward at the same time.