'Sato looked again at the capstone, her expression one of disbelief.'
I know the sentence is perfect both grammatically and semantically because it is from a novel written by Dan Brown. However, it seems more complete to me to write:
'Sato looked again at the capstone and her expression was one of disbelief.'
Or
'Sato looked again at the capstone with her expression being one of disbelief."
What is the grammar rule behind this? Why can you just write " ..., her expression one of disbelief." ?