here's an article from a few years ago talking about xT, nicely illustrated by an excellent mesut ozil pass for arsenal.
https://karun.in/blog/expected-threat.html
whereas xG usually just looks at the shot at the end of the move, this tried to look at each pass or movement in the game and the effect of it.
follow up question for anyone interested enough to read it - can you guess who has the highest xT per 90 in the Cardiff squad this season.....