In order to detect key presses, the keyboard controller will scan all columns, activating each one by one. When a column is activated, the controller detects which rows are "activated".
To step through this procedure, the controller activates column C1 and checks rows R1 and R2:
Figure 3. Scanning Column 1
Neither key A nor C are pressed, so neither row R1 nor R2 is activated. The controller now knows that both nodes C1R1 and C1R2 are deactivated.
Then it activates column C2 and checks rows R1 and R2 again:
Figure 4. Scanning Column 2
Neither key B nor D are pressed so neither row R1 nor R2 is activated. The controller now knows that both nodes C2R2 and C2R2 are deactivated.