To understand zmv ## patterns

This is zmv command to non-recursively replace any number of spaces with a single underscore (thanks to Stéphane Chazelas, who helped me with it):

zmv -- '* *' '${f// ##/_}'

And here is a command from the internet, that seems to do the same thing, but recursively:

zmv -- '(**/)(* *)' '$1${2//( #-## #| ##)/_}'

Could someone explain how does it work? Specifically, how the ( #-## #| ##) part works?