on one's back; face up
Describes a posture of lying or falling backward. Most often used adverbially as のけざまに.
He fell on his back.
I lay on my back and looked up at the sky.
The derivation is uncertain. The kanji 仰け様 suggests a connection to 仰ぐ (to look up) and 様 (manner), but the word is now almost always written in kana.