Eyyyyyy. Resurrecting with new information.
Over the weekend I was privileged enough to get a game in with the creator of "Happiest Apocalypse on Earth". He is an avid fan of the "X CARD" and he described it in a way that clicked well with me.
He said that he finds that players often seem more likely to get grimdark (my fave!) when there IS an x-card on the table because they can do so knowing everyone has the ability to opt out at any time; that the existence of the X-card makes it more okay to make bold/daring IC decisions because theyre comfortable knowing that anyone who doesnt like it can X it.
The X card can make a player less nervous to try things.
I really liked that.