Language Point 36

As it swung back and forth, ...(Para. 14)

Meaning: While it moved back and forward...

back and forth: move back and go ahead or forward

A pendulum swings back and forth.
钟摆前后摆动。

He walked back and forth in his room waiting for the news from his son.
他在房间里走来走去，等候他儿子的消息。