I believe the idea is that you undo their move and they have to play a different move instead. A lot of fantastic chess moments are about one player finding the

I believe the idea is that you undo their move and they have to play a different move instead. A lot of fantastic chess moments are about one player finding the only move that’s good, and high level chess is largely about creating these incredibly tense board positions where there are a lot of possible moves but only a few are safe. For example, Sesse’s analysis of the last championship game http://analysis.sesse.net/ http://analysis.sesse.net/ (can’t link to a specific move, use the bar chart or the Previous/Next hyperlinks to go to “analysis after 58… a3”). This is 50+ moves into a fast paced game and according to supercomputer analysis it is a perfectly tied position. There are two possible moves that keep the game tied or close to tied, and one more possible move that only give up a very slight advantage (0.5 points is roughly “half a pawn’s value” and is close to the limit of what a human player can confidently detect, this moves gives four tenths of a pawn). Every other move gives up 3 points or more (roughly a full piece, like a bishop) - not always immediately, maybe five or ten moves down the track. Nepo, playing white, makes one of those “bad” moves, theoretically losing the equivalent of almost four whole pawns, a terrible blunder in the eyes of the all-seeing computer. But use the Next link to look at the position in “analysis after 59 Qc7”. There is one move, black Queen to g6, that takes advantage of the “blunder”. There is a second move, black Bishop to f8, that completely cedes the blunder and returns to a mostly-tied position. The third best possible move loses 32 points (going from -4 to +28, which is the equivalent of losing both knights, both bishops, both rooks, and your Queen in a single move - the computer equivalent of saying “you lose the game, it just takes 20 or more turns”). Then there’s two dozen more possible moves, all of which literally lose the game on the spot. So to recap: one move gets your opponent ahead, one move keeps him tied, and twenty-four moves loses him the entire game. Being able to cut off your finger to undo and prevent that one good move would change the entire dynamic from “a terrible blunder” to “a bold stroke of pure brilliance”.