Do you just decide to make two moves?
That is you make a move, opponent makes a move, you undo your opponent move. Now whose turn is it? is it your turn? where you have effectively moved twice, your opponents turn? but we can't can just let them make the same move, that goes against the spirit of the thing. So perhaps their turn but they can make any move except for the one you undid.
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”.