Karjakin, who was extremely short of time, did not notice that there was an even better version of his idea--one that would have won on the spot. He could have played
Nakamura-Karjakin after 42...Rd3!! (analysis diagram)
The point is to cover b3 so that White can't recapture with his king after 45...b3+. If 45.Kxd3 b3! queens a pawn. White's rook and knight are so out of position, in fact, that they are helpless to stop 45...b3 regardless of White's next move.