顺水推舟，顺水推船

shùnshuǐ-tuīzhōu, shùnshuǐ-tuīchuán

[push the boat along with the current-make use of an opportunity to gain one's end] 比喻顺应形势行事

天地也!做得个怕硬欺软, 却原来也这般顺水推船!--元·关汉卿《窦娥冤》