Strong hand, strong body, and combat experience, and then Finnick says the magic word. Trident. At that point he's clearly holding Finnick's hand too long in the shake, but mostly because he's clearly a bit awed.

"A trident? I dedicated three months when I was sixteen to trying to find a reasonable method of combat making use of a trident. For all of my work it was just too different from other polearms, given the sorts of creatures on my world, to figure out a way to fully incorporate it into my combat style. Problem was was that, unlike spears, it wasn't easy to wrest the things out of the sort of creatures I was up against. And I could never figure out a way to use them against a human opponent."

Oh, right, hand. Damn, he's... He's excited. was that wrong? Was it wrong to get excited and ask a guy to teach you how to fight with something without a lot of lead up time on having met them? Yes? Well, he didn't care. This was something he wanted to do anyway. Even if they didn't have the proper gear.

"Maybe you could show me some moves sometime."