Hi 64,
Mate in three. Considering what was at stake, the clock ticking and threat of mate in one etc..
White lifts the Rook off the back rank to kick it off which allows another mating possibility for Black.
1.Rc8 Kh7 2.Qh6+
The beauty is in the two variations mating with either Rooks.
2...Kxh6 Rc8-h8 mate. or 2....gxh6 3.Rf5-f7 mate.
I was forced to watch the Peter Svidler and some gooney guy's site (Chess 24)
Peter's a great player and a very knowledgeable commentator (it's just his dreary
voice that put me to sleep, he has to change pitch every now and then.)
When he was shown this possibility by the goon he said no one would do
that in a World Championship play-off game, you play Qg3 within three seconds.
When Carlsen played it he was full of admiration he was not embarrassed
but surprised and very impressed, said that was an incredible move.
An unforgettable move, a real bolt from the blue. It's up
there with Marshall's 23...Qg3 in Levitsky - Marshall 1912.
I wouldn't go that far, there were multiple continuations after Marshall's crazy looking move, all winning but not all led to forced mate.
In Magnus' example after Qh3 there were only 2 continuations, both of which were mate in 1.
It was a nice touch to end the match, but I wouldn't exactly hail it as 'move of the century'.