Hint 1
Black must avoid checkmate.
Hint 2 (a very big hint)
Black's e pawn.
Spoiler
https://www.chess.com/analysis/game/live/122884427351?tab=analysis&move=72
e1=N+ - underpromotion!
Underpromotion is very rare. The key thing is it's a check and a check is a tempo which is a very important concept in chess. Why not a queen? No tempo :( so white has checkmate in 3 starting with Rc8+.
I watched this speed chess game live and the move happened even before I thought about underpromotion. I wonder how long it would have taken me to see it in a slow game.
I had that move in mind, but I didn't see the underpromotion. Brilliant.