Picture the line of people at the edge of a game. They are not playing. They face the crowd instead of the field, they shout the same words at the same time, and they lift each other into the air so the support can be seen as well as heard. Their whole job is to make one side's noise bigger, and nobody expects them to stop and mention what the team did badly.
cheerleading 可以想成場邊那一排人。他們不下場比賽。他們面對觀眾、背對球場，一起喊同樣的話，還把人舉高拋起，讓「支持」不只聽得到，也看得到。他們的工作就是把其中一邊的聲音放大；也沒有人期待他們停下來說說自己這隊哪裡打不好。這個畫面有兩個部分：一是「整齊、賣力的表演」，二是「聲音只往一個方向去」。英文後來就從這兩個部分各長出一個意思。
From the word’s history
Wiktionary gives the formation as cheerlead + -ing, and cheerlead itself is cheer plus lead. The literal build is leading the cheering, which is exactly the picture at the side of the field.
Wiktionary 記的構詞是 cheerlead + -ing，而 cheerlead 本身是 cheer（歡呼）加 lead（帶頭）。字面上就是「帶頭歡呼」，再用 -ing 變成這件事本身——正好就是場邊那個畫面。
Wiktionary, "cheerleading"
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