Back muscles: How do they work? Seriously, though, it took several tries and checking references to get something I was satisfied with.
I'm writing this before I've seen reactions to the previous comic. I can only assume they were 100% positive and everyone liked it forever. For anyone thinking this is a bit much, however... Ye gods, do yourself a favor and don't think too hard about how a lot of people would use this power, because this is super tame and restrained by several magnitudes.
Panel three of this comic is copied and resized from two comics ago. When I made that page, I grouped Sarah's starting postion and the background together, and made all the other Sarahs separate.