on one’s back
Quando garoto eu costumava deitar de costas na grama e olhar as nuvens brancas.
When I was a boy, I used to lie on my back in the grass and look at the white clouds.
Ela nada de costas melhor do que de frente.
She swims better with her back to the camera than with her front to the camera.
Quebrei o recorde da escola no nado de costas.
I broke the school record in the backstroke.
— Moonrise Kingdom (2012)
Eu troquei as taças quando te virou de costas.
I switched the glasses when he turned his back on you.
— A Princesa Prometida (1987)
Vou ficar de costas enquanto você se veste.
I’ll turn around while you get dressed.
He knows how to swim on his back.