You’re missing the double quote before the ‘d’ which means you’re talking about a register.

Ah, thanks. You're right, my brain parsed the gp's comment as simply "dyy".

Just for completion, `dd` deletes the current line in vim. I'm not sure what `dyy` should do, if anything.

Maybe it should remove the top item from the list of copied items? Assuming that vim has one of those (in Emacs parlance it's called the kill ring).

> I'm not sure what `dyy` should do, if anything.

Currently, nothing. `y` isn't a defined motion.