It wasn't an illegal move, it simply blundered a rook. A backwards bishop skewer after bishop f2+, picking up the rook on b6. Carlsen, before he made this blunder, was in fact rook for bishop and two pawns, only one of which was passed. But really, I think Nepo suckered Carlsen into that one. Moving the king forward somewhere is an obvious move for White, and by putting the bishop on e1, Nepo set up the Bf2 skewer if Carlsen moved the king to the wrong square, which he did. Game over.Alex Holowczak wrote:"Can one give Carlsen the benefit of the doubt here? He hadn't pressed his clock, and I am under the impression (without having bothered to check) than in blitz - as opposed to rapidplay - the move is only complete when the clock is pressed. ("Touch-move" may still apply but then he was happy to move his king to another square). So either he was allowed to change his move or at least thought he was entitled to do so."
The move is made when you've let go of the piece.
The move is completed when you've pressed your clock.
So in this case, Carlsen made an illegal move.
Carlsen actually looked at the clock, rather than the board, as he was making the fatal move! Moved the king forward to d4, took his hand off the king, and milliseconds later realised his mistake. His hand was about halfway to the clock when it came back and moved the king to c4. Instinctive reaction, I think, and understandable. After moving the king from d4 to c4, he pressed the clock at the same time as Nepo was pointing at the d4 square and objecting.
Carlsen then looked at Nepo quizzically, said something that might have been "are you sure?" Nepo stopped the clock and pointed at the d4 square again, and at that point Carlsen with a wave of his hand accepted his fate and they shook hands. Annoyingly, someone moved in front of the camera, so half of Carlsen's facial expressions are lost apart from a brief glimpse of him looking rather tight-lipped.
EDIT: Apologies, it was Savchenko that Carlsen was playing in this game, not Nepomniachtchi!