One way to hold this word: a person on a bicycle whose feet suddenly turn the pedals the wrong way. The bike is still pointing forward, the rider is still facing forward, but the effort has flipped direction. Sometimes that stops the bike; sometimes the pedals just spin and nothing happens except the noise. Everything else the word does grows out of that flipped-direction picture — feet moving back while the face stays front, and words moving back while the speaker keeps a straight face.
From the word’s history
The source gives the word as back + pedal, which matches the picture directly: the pedalling is simply happening the other way.
來源資料寫的是 back（往後）加 pedal（踩踏板）。這跟畫面完全對得上：踩還是那個踩，只是方向反過來。
Wiktionary entry for backpedal (source_ref kaikki:3f6f69a190036ae4:1041463625): 'From back + pedal.'
登入就可以看每一支怎麼連回核心畫面。