Black to move. How should black proceed?
1......Bf4 pins the knight and has the latent threat of moving the knight on d5 and delivering mate with Rh1. If Carlsen tried to give the king an escape route by playing 2.h4, black forecloses this by playing 2.....Ne3 covering the g4 square and white will have to give up the queen to prevent mate by Rh1. The most critical line is, though, is 2.f3:
1....Bf4 2.f3 Rc2+! 3.Kg1 Bg3!
And now the threat is Ne3 followed by mate on the back rank. White can try to walk the king over towards the rook, but this won't prevent white losing material, though this line is quite complicated, it is all forced:
4.Kf1 Bb5+ 5.Kg1 Nf4 6.Qb8+ Kg7 7.Qxe5+ f6 8.Qe7+ Kg8 9.Qd8+ Kf7 and suddenly white is all out of checks and mate can't be prevented, only delayed.
Finally, at move 4, white should try a more active plan, similar to what Carlsen did in the actual game but a bit more successful at fighting back:
4.Bc5 threatening his own mating line, but black has a winning ending:
4.......Bh2+ 5.Kh1 Ne7 6.Qd3 Rxc5 7.Kxh2 Nf5 8.b4 Rd5 etc.