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Equal numbers in a line must stand apart — and the bigger the number, the further.
The board is divided into rooms, and a room of n squares holds the numbers 1 to n, each exactly once. The rule that defines this puzzle is the second one: if the same number appears twice in a row or a column, the gap between them must be at least that number of squares. Two 1s need one square between them, two 4s need four, so large numbers push their twins a long way off and small rooms holding large numbers are usually where a grid opens up. Tap a square, then tap a number from the pad below; the erase key clears a square. Every puzzle is generated with exactly one solution and is verified to be solvable using forced moves alone, so guessing is never required. Solve one and the next arrives with less time on the clock, and the clock keeps shrinking without limit, so the run always ends. Your score is the number of puzzles solved.
Start where a room is small and the numbers it must hold are large. A two-square room holds a 1 and a 2, and the 2 alone reaches two squares in each direction along its row and its column — that single placement often eliminates candidates all over the board. A five-square room is far weaker per square, because its 1 barely constrains anything. Scanning for the tightest rooms first is worth more than working left to right.
The distance rule works in both directions, and beginners only use one of them. Yes, a 3 forbids another 3 within three squares. But it also means that if a row already has a 3 near the middle, the only places a second 3 can live are the far ends — and if those cells belong to rooms too small to contain a 3, the row simply cannot hold two, which tells you something about every other room it crosses.
Because each puzzle is guaranteed solvable by forced moves, you never have to branch. If you find yourself weighing two possibilities, there is a square somewhere with only one candidate that you have not spotted. The usual place to find it is the largest number in a small room, or a cell squeezed between two rooms that both already contain their low numbers.
Against the clock, fill in what you are certain of and do not stop to admire it. The grids are small and the chains are short; most of the time lost is spent rechecking a placement that was already forced. Trust the guarantee: if it was the only candidate, it is right.