The example is a good one -- the third cell is set to 300 AND a1 + a2. Then, a1 is set to 51, which automatically sets a2 to 249. At no point was a2 given a for

The example is a good one -- the third cell is set to 300 AND a1 + a2. Then, a1 is set to 51, which automatically sets a2 to 249. At no point was a2 given a formula, it is just constrained by the formulas in other cells.