quick question about carlsen-gelfand

Why is Nh5 "the only move" here?:

I believe probably because of the pressure white had on g7 and f6, and the threat of b5 followed by Ne4. I'm guessing though.

All other moves gives white a winning advantage, where black has next to no counterplay.

A good way to see what the white plan is, is to make it his move. If 24... pass then 25. b5! Be8 26. Ne4! Now, black must either lose a knight or play Nh5 (all other knight moves end in mate on g7), 26.. Nh5 27. Qc7, where white will win the b7 pawn and end up 2 passed pawns up on the queenside. This is winning for white in 999/1000 games on GM level, so accepting that is not good.

Back to the game. Black is preventing the dangerous Ne4 with the bishop. So, white has the threat of b5!

Tactically, black can defend with 24.. Nh5! 25. Qe5! This forces a good endgame for white. 25... Bf6 26. Qxh5 Bxd4 27. Rxd4 Qxc3 28. Qa5!! The queen is extremely well placed on b6, and with tactical means Carlsen places it there. Back-rank mate theme in (super)GM games!