There's kinda a secret to this, it's dangerous because pain is an indication of something being bad for you, but you let your hand go forward, and only concentrate on lifting with your elbow and you too can lift it.
Wrist abduction, a movement you very rarely do with no weight (unless jacking off with weird moves), let alone with a 10kg hammer.
The forearm muscle that controls it it's called extensor carpi radialis longus. Blacksmiths (which the old dude probably is) strengthen this muscle a LOT through repeated heavy use while hammering, way more than weightlifters.
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