A cool video from NGK and an interesting discussion at bobistheoilguy board.
https://bobistheoilguy.com/forums/ubbthreads.php/topics/4594452/Pre-Ignition_and_Knock#Post4594452
https://bobistheoilguy.com/forums/ubbthreads.php/topics/4594452/Pre-Ignition_and_Knock#Post4594452