One long board, balanced on a point in the middle. It has two ends, and only two. Push one end down and the other end must come up; a moment later they have places. Nothing here climbs on its own, and nothing stays at the top. That shape sits behind every use of the word: two sides taking turns, over and over. First it is the real board in the park, and then it is prices, moods, scores and votes keeping the same rhythm.
Our source says the word is probably an imitative, reduplicated form for rhythmic back-and-forth motion, in the same family as teeter-totter, zigzag and flip-flop, and that the verbs see and saw likely influenced its shape. If that is right, the sound of the word is already doing the rocking: see - saw, see - saw. It is a likely account, not a settled one.
來源說這個字很可能是模仿「來回擺動」的疊字詞，跟 teeter-totter、zigzag、flip-flop 屬於同一類；也可能受到 see 和 saw 這兩個動詞的形狀影響。如果這個說法成立，那這個字的聲音本身就在搖：see—saw、see—saw。來源用的是「可能」，所以這是一個有依據的推測，不是定論。
Wiktionary/kaikki extract supplied in this packet (source_ref kaikki:3f6f69a190036ae4), field etymology_text for seesaw.
Two branches
公園裡真的那塊板子
這一組就是核心畫面本身，還沒有進入比喻。一個是那塊板子，一個是人在板子上一上一下的動作。兩者的主詞都是看得見、摸得到的東西。
noun · A2
A long board that rests on a support in the middle, used for play in a park or playground: a child sits on each end, and when one end goes up, the other end goes down.
蹺蹺板
抽象的事情一上一下、輪流換邊
把板子拿掉，只留下那個節奏。只要一件事有兩端，而且兩端一直輪流佔上風，英文就可以叫它 seesaw：價格、心情、比分、票數都算。這是一種畫面上的借用，不是說這些意思在歷史上比較晚出現。
noun · B2
A repeated up-and-down or back-and-forth change: something rises and falls and rises again, or two sides take turns being ahead. It is used about numbers, prices, feelings, and how a game is going.
反覆起伏