Too often in games, game makers punish players inadvertently by making them game too hard. Obviously, there are cases where making the game hard is in pursuit of fiero or hard fun (see the Four Fun Keys). However, what about the player who is trying hard to play but lacks the motor skills to master the game's demands? Should this player walk away from the game, frustrated and angry at not being able to complete a game that they were previously enjoying tremendously? How many players fail to complete a game they love because of a boss battle they simply cannot win, even using a cheat guide? If that player hasn't got a 10 year old handy to play that boss battle for them, they will quit in frustration or simply lose.
Frustrated and angry is no way to leave a player. Players should leave a game with a sense of satisfaction and having accomplished something. To consign a player to the world more frustrated than when they went into the game is not a way to continue to make money either.
The Buster Principle is simple: be kind to your players. In the event that a player has clearly tried repeatedly to succeed at a task (something easily tracked in modern games), try making the task just a little bit simpler. It may not even be enough to be perceivable to the player; all that much better. A small downward adjustment in the difficulty may be the difference between an unhappy player throwing the controller across the room and an ecstatic player rejoicing and pumping their fists with a sense of accomplishment.
The idea here is to not make the game so hard that the player gives up. This is not in any way to suggest that all games be made easy. It is simply a tenet that says that making the game inordinately hard is simply showing the player that the game maker is smarter/faster/better than they are. Infocom games were well known for how impossibly hard their puzzles were. So hard, in fact, that it led to an entire product line known as Invisiclues, clue books that came with a special marker for invisible ink that would allow a player to get a small hint or the outright answer to a puzzle by using the pen on the numbered clues. The clues were increasingly direct, the first being tiny and obscure.
Games have come a long way since then and now that game makers can easily detect how much a player is working at solving a problem, better solutions to the problem are almost trivial. In short, don't torture players. Throw them a bone now and then to reward them for continuing to try.