A value spreads from the top-left corner. Each step, every cell adds 1 to its value (mod 10) — this is the additive phase (steps 1-3). At step 4, the pattern REVERSES: every cell subtracts 1 (mod 10) instead. From step 5 onward, it subtracts again. Grid 4 (position 3) is masked — it is the first subtraction step, so its values equal those of step 3 minus 1 (mod 10).