From Bulbapedia, the community-driven Pokémon encyclopedia.
The Action Replay is a cheating device brand made by Datel that is used for games and systems of all kinds. It is not licensed or endorsed by any game or console manufacturer.
While the GameShark was the most widely used cheating device in the late 90s, it was actually a rebranded Action Replay. However, Interact, the company who distributed GameSharks, went bankrupt. After that, Mad Catz bought the name GameShark, and Action Replays were then sold to the public by their original name.
Use and function
Cheating devices such as the Action Replay are primarily used to enable, disable, or modify sections of a particular game's code. By intercepting game code transmission between the game's software and the system's hardware, Action Replay devices can change the gaming experience. The use of cheating devices to modify Pokémon games has been a popular practice since the games' release in the mid-nineties. Popular applications of the device include capturing unobtainable Pokémon, acquiring normally unobtainable items or mass quantities of items, and modifying Pokémon stats.
Although Action Replay can be helpful, it is also known to freeze the game occasionally and to cause minor problems. Although codes that simply allow users to change Pokémon, levels, or stats tend to be harmless, others like Walk Through Walls or codes rewriting major events in the storyline may result in minor game glitches or corrupt the entire save file.
In 2005, Datel released a new device for use with both the Nintendo Game Boy Advance and Nintendo DS called the Action Replay Duo (also known as the Action Replay Max). This new device enabled users to modify GBA games like the ordinary Action Replay for GBA; however, it also had the ability to save DS games onto a computer using a USB cable. Using this method, users could backup and restore saved game files. However, the device does not recognize any Pokémon save data located in its slot, and has caused much outrage among fans for this reason. Despite numerous updates to the device, there has been no explanation given as to why the device ignores Pokémon games.
*Indicates glitches which have been officially acknowledged