How do moves like Metronome work?

In my experience, quite well.

Edit: I have just been informed by my editor that I’m only allowed to make that joke once on this blog before “it stops being funny,” and therefore, I’m to offer an actual response to this question.

The truth of the matter is no one’s quite sure. The going theory is that those who use Metronome tap into some ancient gene (passed down from Mew, supposedly) that allows a pokémon to use any move in existence. This theory hinges on the idea that most elements and techniques are actually hard-coded into a pokémon’s DNA; it’s just that pokémon can’t use just any move in existence due to the physical limitations they’ve gained through adaptation and the slow, progressive sort of evolution. However, Metronome allows a pokémon to override these blocks and call upon elements and techniques just once, at random.

The other theory, which is slightly less popular (although it makes somewhat more sense to me, personally), is that the pokémon doesn’t call upon a random technique but rather the move itself does. That is, the user merely conjures an untamed sort of energy that manifests into a random move, occasionally even puppeteering its own user in order to do so. This explains why a pokémon is often incapable of remembering how to use whatever came out of Metronome until they attempt the Metronome technique again (and sometimes not even then).