S.O.S battles are a much faster way of EV training than super training. You only need to knock out 7 Pokemon using this method to elevate one of the EVs of a Pokemon to 252. Here's how:
Pokerus
If you have the Pokérus on a Pokémon, you have probably noticed that it will spread to the other Pokémon in your party and when you level one of those Pokémon up by battling, their stats will increase higher than usual. The reason for this is because the Pokérus doubles the Effort Points you've obtained.
Battle Items
The Power Items are items that have often been used to aid in the training of Pokémon. From their conception to the end of Generation 6, they always added 4 EVs of a specific stat whenever you defeated a Pokémon, regardless of what Pokémon you defeated. Pokémon Sun & Moon however has significantly buffed them. Instead, each of the Power Items will give 8 Points in the stat after battle. This makes them a significantly better option over the Macho Brace. They can be purchased in the Battle Royal Dome
S.O.S battle chaining
As the Pokémon calls for help, the Effort Points yielded from each Pokémon are multiplied by 2 which results in the Effort Values you receive getting increased. As such, with this, a Power Item and PokéRus, you can get 36 Effort Points from one Pokémon (((1+8)2)2) being defeated and max a stat out in 7 KOs in a chain.
Even if you don't have pokerus on your Pokemon, you only need to knock out 14 Pokemon, which is still much faster than super training. All you need to do for the slower method (14 KOs) is buy the power item you need, buy an adreneline orb from any Pokemon centre and do the S.O.S chain battle. And one of your Pokemon will catch the pokerus eventually, so you can do the faster method instead.
And if you can buy 6 of the power item that corresponds to the EV you are training, and you have pokerus on the 6 Pokemon you want to train, you can train the EVs of 6 Pokemon at once:
EXP share
In Pokémon X & Y, the item is now a Key Item and can be turned on or off. Doing so results in all Pokémon receiving the EXP. They receive 100% of the EXP if in battle, and 50% of the EXP if they were not in battle.
EVs
EV stands for “Effort Value.” They determine what stats your Pokémon gets upon levelling up and are gained from battling any Pokémon you come across that award Experience points.
Source: Serebii
Go to the bottom of this for a table of where to do S.O.S chain battles for each EV: http://www.serebii.net/games/evs.shtml
