|Sally Simpson: As Honza states 51...Rb1 was a blunder.
Let's look at it from Stein's side of the table. (Black to play)
click for larger view
The only thing that can screw up the plan of getting that White g-pawn off the board and going into a KRN v KR ending is a possible Knight Fork.
But looking at the board the Knight Fork square for g5 and f8 is e6 (remember the board is upside down, we are seeing what Stein saw.)
Everyone knows the pattern, if a Knight is two diagonals away from a square then it takes the Knight at least four moves to get there.
No danger there so Stein played 51...Rb1 (51...Ke8/g8) then he saw what Kholmov saw. There is another g5 f8 Fork with the Knight on h7 and it can get to there in two moves. OOPS!
A great fighting game this one.