Heres what I've told numerous people about Ev Training. Hopefully it can help you
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Well, first off, Ev Training is training pokemon in a specific stat, such as Attack and Speed. Each time you battle a pokemon, they give off what we call "Effort Points". Whenever you kill one, your pokemon will gain the Effort Points that are given off by the pokemon. For instance, when you kill a Bidoof, your pokemon will gain 1 HP Effort Point. These effort points will eventually change how much you will gain in a stat when you level up. So lets say you kill 100 Bidoofs (don't have to, I'll tell you in a little bit), you might gain +6 in HP then +3 when you level up. It might not seem like much, but when you get near Lv.100, you'll see a huge change in the stats. This is why you don't want to battle random pokemon, because if you battled a Bibarel, a Bidoof, and a Geodude, you would gain 2 Attack Effort Points, 1 Hp Effor Point, and 1 Defense Effort Point.
All together, a pokemon can gain 510 Effort Points; 252 max in two stats, and 6 in another (It does not have to be 252, it can be any number under that, but if you want Max Attack, you'd want to use 252). So if you see spreads such as 252 Attack/252 Speed/6 Hp, it means that they trained their pokemon to battle possibly 252 Scythers, 252 Starly's and 6 Bidoofs. Now you don't have to battle 252 Scythers. Thats what the Power Items are for (Power Bracer, Power Anklet, Power Lens, and others). Each Power Item will give you a +4 Effort Points in a specific stat (For instance, if you battle a Starly and have the Power Anklet attached, instead of gaining 1 Attack Effort Point, you'll gain 5 Attack Effort Points). So, instead of battling 252 Scythers, you only have to battle 50.4 Scythers. Also, if you have Pokerus infected, it will diminish the amount of pokemon you have to battle. If you have Pokerus infected, it will double the amount of Effort Points you obtain. So lets say your battling a Bidoof and you have the Power Weight Attached, in total, you will gain 10 Ep's instead of 5, which means you only have to battle 15 Bidoofs! ({1 [Ep given off] + 4 [Power Item]} x 2 [Pokerus])
Please notify me if you have any other questions, I might have missed something ^^