with one's back turned; on one's back
"¿Por qué las ardillas nadan de espaldas?""Para no mojarse las nueces."
“Why do squirrels swim on their backs?” “So they don’t get their nuts wet.”
Por poco me caigo de espaldas de la silla al ver lo que pasó después.
I almost fell backward out of my chair when I saw what happened next.
El viejo estaba de espaldas a mí.
The old man had his back to me.
Piotr hizo una reverencia y salió de espaldas por la puerta.
Piotr bowed and backed out through the door.
He was lying on his back.
Estaba tendido de espaldas en la playa.
He was lying on his back on the beach.