This variant was invented in 1973 by Bernd Eickenscheidt and B. Schwarzkopf. Postal tournaments in this game are held in NOST. Originally, the idea was mainly used in fairy chess problems, but it was soon found out that the game is also nice for playing.
The only difference with the rules of orthodox chess, is that pawns may also promote on the sixth and seventh rank. When a player moves a pawn to the sixth rank, it may choose to promote the pawn, or may decide to keep the pawn as a pawn. (Usually, one would choose the first option, but one might for instance want to avoid stalemate, or leave the option to take a knight open.) When the pawn remains a pawn, the same choice is there again when the pawn is moved to the seventh rank. When a pawn is moved to the eighth rank, it must be promoted.