"Chaining is useful for EV-training, because a called ally doubles the amount of EVs it gives when defeated. For example, a Caterpie normally gives 1 HP EV, but a Caterpie called as an ally in battle will give 2 HP EVs.
With the Power Items now giving 8 extra EVs (4 extra EVs previously), a stat can be maxed at 252 EVs with an ally chain of 14, or 7 if the training Pokémon has Pokérus. This works as follows:
A Caterpie gives 1 HP EV. This becomes 9 HP EVs if the training Pokémon is holding a Power Weight (1+8). If the Pokémon also has Pokérus, the EVs are doubled to 18. And if the Caterpie is a called ally, the EVs will be again doubled, to 36. Defeating seven called Caterpies would therefore give 252 HP EVs (7x36)."