I heard of it and want to try it. What is it exactly?
Any Pokémon that faints is considered dead, and must be released or put in the Pokémon Storage System permanently.
The player may only catch the first Pokémon encountered in each area, and none else. If the first Pokémon encountered faints or flees, there are no second chances. If the first encounter in the area is a Double Battle in dark grass, the player may choose which of the two Pokémon they would like to catch.
While not exactly a definite rule, the general consensus is that players must also nickname all of their Pokémon, for the sake of forming stronger emotional bonds.
Also not a definite rule, but the general consensus is that a black out/white out is considered to be "game over", even if there are Pokémon left in the PC
Hope I helped!