The Power Anklet raises how much Speed EVs you collect. So unless you are EV training this Item is useless. When this item is held by a Pokémon, the holder's Speed is reduced by 50%. For every battle it has, in addition to the Effort Values gained in the battle, it will gain 4 Speed Effort Values. Shedninja's Base Speed is 40 so if you give it a nature to Raise Speed and you give it 252 Speed EVs it will have over 40 Speed Stat.