How to solve an empty rectangle in Sudoku
An empty rectangle uses a digit confined to one row and one column of a box, plus a two-place link elsewhere, to remove the digit from one cell.
How it works
In a box, all the spots for a digit lie along one row and one column of the box, so the rest of the box is an empty rectangle for it.
Add a link in another column in which the digit has only two places, one of them in the rectangle's row. Either the digit sits in that link's end in the row, which pushes the box digit into the rectangle's column, or it sits at the link's other end. Either way, the cell in the rectangle's column that shares a row with the other end cannot hold the digit.
How to spot it
Look for a box where one digit forms an L or a cross, then check the columns and rows beside it for two-place links.
A worked example
This is a step from a real puzzle, a moment where the next move needs an empty rectangle.
W are the digit’s cells in the box; A and B are the two ends of the link. Struck-through digits are the ones that can be removed.
In box 1, every 5 is in column 3 or row 3 (an empty rectangle). In row 7, 5 has only two places: R7C3 and R7C8. If R7C3 is 5, it clears 5 from column 3, which leaves the box's 5 in row 3. If it isn't, R7C8 is 5. Either way R3C8 sees a 5, so remove 5 from it.
Where it fits in Rocksled Sudoku
Puzzles that need an empty rectangle are never rated below Very Hard. Each step of this kind adds 34 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.