One way to picture it: someone holds a thing that could hurt you — a secret, a proof, even your own guilt — and won't put it down. "Do what I want, or I let this out." The force is never in a fist; it is in what they are holding.
手上握著一樣會傷到你的東西，然後不放手——「你照我說的做，不然我就把它放出去。」重點不在暴力，而在那個被握住的把柄：一個秘密、一段醜聞、一份證據，或者一份你欠對方的人情。對方不必動手，只要讓你一直感覺到那樣東西還在他手上，你就會照做。
From the word’s history
Originally the tribute that English and Scottish border dwellers paid local chieftains for protection — mail here is an old word for rent or payment, black marking it as the illegitimate kind. From the start it named coercion-by-holding, not robbery.
這個字原本指十六世紀英格蘭與蘇格蘭邊境居民付給當地首領的「保護費」——mail 在此是古語的「租金、款項」，black 則與正當繳納的「白錢」相對，指這種被迫上繳的黑錢。也就是說，它從一開始就不是搶劫，而是「你付了才會平安」的持續脅迫——這正是現代用法裡那份「握著不放」的味道。
Wiktionary, "blackmail" (black + mail "a piece of money"; compare Middle English blak rente)
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