A retro move is the last move played to reach the current position.
Example
Play 1.32-28.
The move 1.32-28 is black, but not bold, it is a retro move. The current position arises 603139 times from the position before 32-28. The percentage score of the position before 32-28 is 49: 100 minus 51 (which is the percentage score of the begin position).
