。
She did a handstand.
Literal
She [topic-は] handstand [object-を] did.
逆立ち is built from 逆 ('reverse, upside down') + 立ち (the masu-stem of 立つ 'to stand'), giving literally 'reverse-standing.' The same noun gets verbed via Xをする — the all-purpose 'do X' construction that turns countless nouns into verb phrases (運動をする 'exercise,' 試合をする 'have a match'). The play of 立つ ('stand') in compounds is common: 逆立ち (handstand), 棒立ち (standing stiffly), 仁王立ち (standing imposingly like a Niō guardian).