Here is the golden rule: If you cannot articulate why a number goes in a cell, you are gambling.
He looked at the fifth row. A 1 was already locked in the fourth column. He looked at the ninth column; a 9 sat perched at the top like a gargoyle. By the logic of the 159 Rule , the fifth column acted as a compass. If the fifth column held a 2, then the digit 5 had to live in the second cell of that row.