Anti-Knight Sudoku Explained in One Short Example
A 6 sits in the corner of a 3x3 grid, and the two cells a knight's move away light up. A second 6 in one of them breaks the rule, and a 3 in the same cell is fine.
Transcript
The rule
Anti-knight sudoku adds one rule to the whole grid. Two cells a chess knight's move apart (two steps one way and one step to the side) can't hold the same digit.
Inside a box, digits already can't repeat, so the rule does its work where it reaches into neighboring boxes. Each placed digit rules out a few extra cells beyond its row, column and box.
Worth memorizing
It reaches into neighboring boxes, which is where it bites.
In a box, a digit on the edge rules out two or three cells in each neighboring box.
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