noun
floating on one's back
Refers to the act of floating face-up in water, often used in swimming or survival contexts.
プールで浮き身の練習をした。
I practiced floating on my back in the pool.
背浮き also means floating on one's back, but 浮き身 is a more general term for the floating posture itself.
Compound of 浮き (uki, 'floating') + 身 (mi, 'body'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning of floating on one's back.