How to solve a two-string kite in Sudoku
A two-string kite is a digit with exactly two places in a row and two in a column, tied together in one box, which clears it from cells that see both far ends.
How it works
The strings are a row and a column where the digit has only two places each. One place from the row and one from the column sit in the same box, which ties the strings together. Those two cells see each other, so they cannot both hold the digit.
So one of the two far ends holds it: if one far end is not the digit, its tied cell is, which rules out the other tied cell, so the other far end must be. Any cell that sees both far ends cannot hold the digit.
How to spot it
Look for a digit with two spots in one row and two in one column, where a spot from each lands in the same box (but not in the same line).
A worked example
This is a step from a real puzzle, a moment where the next move needs a two-string kite.
F are the two cells tied together in a box; R are the two far ends, one of which holds the digit. Struck-through digits are the ones that can be removed.
4 has only two places in a row (R5C2, R5C5) and in a column (R6C6, R7C6), and R5C5 and R6C6 are in the same box, so they can't both be 4. That leaves R5C2 or R7C6 holding it: if R5C2 isn't 4, then R5C5 is, which rules out R6C6, so R7C6 must be. Remove 4 from cells that see both ends: R7C2.
Where it fits in Rocksled Sudoku
Puzzles that need a two-string kite are never rated below Very Hard. Each step of this kind adds 38 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.