to move the pedals (= parts you operate with your feet) backwards on a bicycle

Some types of bike have brakes that you operate by backpedalling. 有些自行车通过倒蹬脚踏板刹车。

to change an opinion that you had expressed before, or do something different from what you had said you would do

As soon as I said I thought she was wrong, she started backpedalling. 我一说我觉得她错了，她就开始变卦。

He said he'd help, but now he's starting to backpedal (on his promise). 他说过会帮忙的，但现在开始变卦了。