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The reason the glitch works is because of what happens with the game when you teleport away from a battle. The game switches into "battle mode", reading code designated for when the player is battling, despite the player not actually being in a battle, which is why the start menu won't load (you can't pause mid battle). The data for the special stat of the last pokemon fought happens to correlate with the index number of the pokemon later encountered, the slowpoke shown will always have a special stat of 21, 15 in hexadecimal, which happens to be mew's index number. When the player returns to the route after this data appears (after getting in a battle), the game thinks that the player should be in a battle on this route, and boots up a battle with the pokemon with the corresponding index number, in this case mew. Fun fact, the level of the encountered pokemon is dependent on the last fought pokemon's attack stages, defaulting at 7. If the player were to use growl on the slowpoke 6 times, he would see a level 1 mew. Due to the way gen I calculates exp for medium-slow exp rate pokemon such as mew, the mew would actually have less than 0 exp, which would roll over if it didn't gain enough exp to get above 0, instantly levelling to level 100. Charmander starts with growl, and is therefore the best starter, especially due to being able to skip the first gym anyways.
tl;dr tricking the game, CHARMANDER DA BEST
