Push the passed....King?!

We all know the importance of pushing passed pawns, it's actually something I've been trying hard to focus on in my games.

Then lo and behold this position crops up, where my central passed pawn gracefully makes up for the absence of two of his precious wingers.  Rather than lunging for the line, however, the coup de grâce is delivered by other more regal means.

Can you spot the winning 🤔 step?  It's Black to play and win.