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.
From the word’s history
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, "seesaw"
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