Picture a board fixed at one end with the other end hanging free. You step out, press down hard, and the board bends under your weight. Then it pushes back and throws you up, and in that same second you have already left it behind. Two things matter in this picture: the push is short, and you do not stay on the board.
From spring + board: the board that springs. The compound says exactly what the object does, and both later uses keep it.
這個字就是 spring（彈）加 board（板子）合起來的，字面是「會彈的板子」。合成的名字把東西的功能直接說了出來，後面兩種用法也都還帶著它。
Wiktionary entry for springboard (packet source_ref kaikki:3f6f69a190036ae4:1223408526:19350)
From the picture
把人往上送的那一下
把板子的兩個特點原封不動搬到人生上：推力很短，而且你會離開它。所以它講的是起步的那一下，不是一路陪著你的幫忙。
noun · B2
Something that gives you the push you need to start something new, or that makes something grow much faster — a job, a show, a contest or a piece of news that sends you on to bigger things.
跳板；起步的助力
noun · B2
A strong, bendy board that throws you up into the air when you jump down on it. One end is fixed and the other is free, and it is used for diving into a pool and for jumping in gymnastics.
跳板（跳水、體操用）