One way to hold propel is a force sitting behind something and sending it forward. The force is not a single tap; it works on the thing and the thing travels. Put a motor behind a boat and the boat crosses the water. Put a hand on a person's back and the person goes through the door. Put one lucky event behind someone's life and they end up somewhere they were not before. The thing being moved changes from a hull to a body to a whole situation, but the picture stays the same: power at the back, movement at the front.
propel 可以想成「一股力量待在後面，把東西往前送」。那不是輕輕碰一下，而是持續出力，東西真的走了出去。船後面裝馬達，船就過得了海。手放在人的背上，人就穿過了門。某件事出現在一個人的人生後面，他就被送到原本不在的位置。被送的東西從船身換成身體，再換成整個處境，但畫面沒變：力量在後，移動在前。
From the word’s history
Latin propello is pro- forward plus pello to push, so the word literally means to push forward.
拉丁文 propellō 由 pro-（向前）和 pellere（推）組成，字面就是「往前推」。這個組合跟今天三個用法的畫面完全一致。
Wiktionary, "propel"
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