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The biological hammer is a modified mouthpart--the 2nd maxilliped. A strike with this hammer can produce a cavitation bubble. The mechanics are complicated, but the results aren't: these little bastards can crack glass in an aquarium. Or, a clam shell.
#functionalmorphology
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Their cousins have a similar but different adaptation. Instead of a superfast hammer, they have superfast spearing claws. These are superficially similar to the claws of a praying mantis, but not homologous. They are often called 'thumb-splitters' by those who fish for a living.
#evolution
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Did I mention they can see more colors than you or I? They can. Humans have three types of photoreceptor cells. Mantis shrimp have between 4-5 TIMES MORE. That would be between 12-16 types of photoreceptor cells. There is also evidence that they can see polarized light. I'm jealous.
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But really, as with all things, mantis shrimp are best explained by @theoatmeal in the form of a cartoon: https://theoatmeal.com/comics/mantis_shrimp
This IS required reading.
Below is a picture I took of mantis shrimp for sale in a Hong Kong fish market (2019).
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@Thomashegna I was thinking about The Oatmeal when I heard about this: https://www.bbc.com/news/world-europe-63996982

@nerdbert @Thomashegna That was just horrifying! One of those just because you can, doesn't mean you should examples of human hubris.

@stsparky confession time: I swiped the picture from Wikipedia. I've seen them in tanks, so I know it is possible.