This glitch is a method of cloning Pokémon in Pokémon Diamond, Pearl and Platinum and Pokémon HeartGold and SoulSilver, performed by preventing the game from properly saving. For this reason, it is still unreliable, and details on its mechanism and safety are still yet to be found.
1. Have the Pokémon to be cloned in your party, and save the game.
2. Place them in the PC, and then begin saving the game again, but shut off the system or eject the cartridge before the process is complete.
3. If you are lucky, the box data will have been saved, but your party data will remain the same as it was in the previous file, giving you two copies of each Pokémon that was in your party at the start.