Opening safe cells
Level: Basic
If the adjacent flag count equals the number, the rest are safe.
If the number of flags (confirmed mines) already placed around an open number cell equals that number, then the remaining closed neighbors hold no mines. You can open them all with confidence.
For example, if a cell showing 2 already has two flags around it, every other closed cell next to it is safe.
Alternating between 'confirming mines' and 'opening safe cells' is the basic rhythm of Minesweeper. With the chord feature, you can open all these safe cells at once.
Related techniques
Basic
Flagging certain mines
If the closed-neighbor count equals the number, they're all mines.
IntermediateChording (batch reveal)
When number = flag count, open the remaining neighbors at once.