Step 19 · Master
What is 2-String Kite? How to Solve Killer Sudoku
When a digit has only two spots in one row and two spots in one column, and those lines meet in one box, you can remove that digit from the cell that sees both free ends.
What it is
Use one digit, with one row and one column where it has only two spots (the two strings of a kite).
One end of the row and one end of the column sit in the same box. They can't both be the digit, so one of the other two ends (the kite's tips) must be.
Any cell that sees both tips can't be that digit.
How to spot it
Pick one digit, find a row and a column with exactly two spots each, and check whether one end of each meets in the same box.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
Think about 5 only. In row 9, 5 fits only in R9C5 or R9C8; in column 4, only in R7C4 or R6C4. R9C5 and R7C4 share box 8, so they can't both be 5. That means R9C8 or R6C4 must be 5. Any cell that sees both (R6C8) can't be 5.