Knees and hips break at the same time. Knees may go slightly past the toes depending on the stats of the lifter.
The diagnostic angles are different, because of the need to keep the bar over the mid-foot balance point. This means the musculature involved is different (more posterior chain in low bar SS style), and that hip drive is possible in the low bar squat.
You read this yet?
http://startingstrength.com/articles/squat_rippetoe.pdf