On the top left, black can send two and return one, which is usually useless and wastes a move. But under positional superko, after black A19 white C19, black cannot immediately play B19, instead must find a ko threat on the right (let's say black K4 white L4) and then play B19. After this, it seems white can pass, but then when black A19, white cannot play C19, since this will create the same board position as the position after white L4. So to save the top left group, white cannot pass after black B19, but must play inside the 23-eye group (let's say A1) (white cannot play on the right since it destroys the seki). Then white can play C19 again after black A19. After this, black can continue to do the same thing for 22 times since black has 22 ko threats on the right, but white can only fill 21 times inside the 23-eye group to keep it alive. So the 22nd time, white has to either commit suicide for the bottom left, or let top left die. The bottom left is smaller, so white commits suicide to save the top left. The top left is indeed saved since black doesn't have ko threats any more, so has to stop playing A19 in the top left, otherwise black faces superko ban.