Forum:How does the Xi collapsing function work?

Disclaimer: This post is not about Adam Goucher's Xi function!

I have seen the Xi function rarely around the wiki as a sort of ordinal collapsing function that uses K (the first weakly compact cardinal) as a diagonalizer. How exactly does this function work? And how does it relate to the \(\psi\) and \(\chi\) functions?

One more sidenote, I don't know how to put actual greek letters into a post like this (instead of english words like "psi"), can someone please fill me in?