I think if you want to really focus on your lower body, then you shouldn't be using your arms for momentum. That seems to me like the people who use full body momentum to do biceps curls, it takes the focus off the part of your body that should be working.
I like keeping my hands behind my head because I tend to collapse my upper body as I get tired (which I do, very quickly, doing the BWM) and it's easier for me to remember to keep my shoulders back & chest up if I've got my hands behind my head. If you can keep good form on this with hands on hips, then you should be fine.
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