take a look at mark's videos on youtube where he talks about hip drive. the essence of the low bar squat is the hip drive, and back angle is really secondary. your back will bear more of the load, so it's important to keep proper spinal flexion. but the crux of the movement when coming back up is to drive the sacrum.
for me, the advantage of low bar squats is that they're just way easier on the knees if you're doing the properly. if you have back issues though, it may be easier for you to stick with olympic style squatting.
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