The Dojo effect is the result of the ability of widespread internet access to affect the way a game is played. The term originally related to the game Magic: the Gathering, but has since spread to apply to any game in which study and research on the internet can alter and improve gameplay. In essence, it is an example of the internet used to further a metagame.
The effect is named after a website called The Magic Dojo, which was extremely popular among the Magic: the Gathering playing crowd in 1997 and 1998. Magic is a card game in which players are allowed to pre-select a deck of sixty cards from which to draw their hands and play; players compete against one another using their specially constructed decks. Naturally, this led to a lot of ideas about which decks were better than other decks and, over a matter of time, some decks would become more popular than others; the effect of this is that decks that were able to defeat the popular deck would come into vogue, and so on.
The origin of the effect can perhaps be directed to chess gambits, where players would memorize particular hypermodern and classic strategies. However, as with the Dojo effect, the separation between the good player and the great player would come with how one could use and adapt these strategies and create new ones. The ever-so-wise Sylvar pointed this element out to me.
Anyway, with a large percentage of the Magic-playing community accessing the website and discovering which decks were in vogue at any given moment, tournaments began to react quite a bit to what was stated on the internet. If The Dojo and other internet sources were saying that the past week had shown one particular strategy to be popular, another strategy would very quickly become popular.
The Dojo effect has two immediate effects on whatever game it is applied to, one positive and one negative. The negative effect is that someone new to the game can simply memorize a particular strategy off of the internet and apply it almost mindlessly, thus appearing much better than the player actually is. The positive effect, though, is the wide availability of these strategies, which thus encourages the game to evolve; if a particular strategy is known to everyone, then it becomes less effective and thus encourages innovation and strategic thought.
The Dojo effect, to a certain degree, has become widespread in many other games with the increasing acceptance of the internet across the country. Chess and especially contract bridge in particular have been influenced by this, with an increased availability of new chess opening strategies and new bridge bidding and playing strategies. It has also had a lesser but still profound effect on other games such as go and shogi.
The Dojo effect is essentially the result of a widespread medium of information sharing. People are interested in sharing information both to help themselves and to help others, and with a worldwide audience, information such as gaming strategies spread very quickly. As a result, there is a profound effect on the games being discussed, and that, in a nutshell, is the Dojo effect.