sail with the wind at its back
Nautical term for a sail that is filled from behind, pushing the boat forward.
We sail with the wind at our back, the sail filled from behind.
aback
Adverbial use describing a sail pressed back against the mast by a headwind, often unintentionally.
The sail was taken aback by a sudden headwind.
Compound of 裏 (ura, 'back, reverse') and 帆 (ho, 'sail'), literally 'back sail'.