Chording (batch reveal)
Level: Intermediate
When number = flag count, open the remaining neighbors at once.
Chording is the technique of pressing an already-open number cell to reveal its surroundings all at once. It only works when the number of flags adjacent to that number is exactly equal.
When the condition is met, all the remaining closed neighbors are treated as safe and open in one go. It's far faster than clicking them one by one, and it's a key technique for cutting your time.
But if you chord while a flag is misplaced, even unsafe cells will open and a mine can go off. Only use it when your flags are certain.