TM51 Ally Switch can be bought for 48 Battle Points (BP) at an Exchange Service Corner within the Battle Subway. The Battle Subway is located in the center of Nimbasa City, and you’ll be able to reach Nimbasa City after defeating Gym Leader Burgh in Castelia City.
The Battle Subway has seven subway trains that hold a different style of battle. BP is awarded to you based on win streaks & the difficulty of each battle in the Subway.
TMs and other items can be purchased with BP at an Exchange Service Corner on each subway.
And as for the move itself:
Ally Switch is a non-damaging Psychic-type move that switches the spots of the user and the ally that is targeted. It can only be used in Double and Triple battles.
TM51 Ally Switch Location (Step-by-Step)
Step 1: Starting at the Nimbasa City Pokémon Center, head east.
Step 2: The large building east of the Pokémon Center is the Battle Subway – enter the building’s front entrance.
Step 3: The entrance to the Battle Subway leads to a circular platform with seven different subway trains.
Enter any subway you’d like – they all have an Exchange Service Corner.
Step 4: After entering the subway, you’ll see the Exchange Service Corner in the northwest corner. Speak to the clerk on the left.
Step 5: TM51 Ally Switch can be purchased from this clerk for 48BP.
Along with TM51 Ally Switch, the clerk also sells the following TMs:
|TM23 Smack Down
|TM34 Sludge Wave
|TM77 Psych Up
Ally Switch Details + Uses
|Ally Switch Move Details
Ally Switch is a Psychic-type Status move that can only be used in Double or Triple battles.
The move targets an ally and both Pokémon switch places. If the user of Ally Switch is in the middle of the field in a Triple battle, the move will fail.
While very situational, Ally Switch can be a very effective way to throw off your opponent.
It can be used to put your Pokémon at a type advantage, therefore negating the effectiveness of your opponent’s attacks. It can also be used to set-up certain abilities for your Pokémon, such as Flash Fire if you’re expecting attacks of a certain type.
Unfortunately, Ally Switch can only be learned by a limited group of Pokémon.
To learn the move, the candidate must have Psychic typing. And it’s rarely learned by level up, so nearly every Pokémon must be taught Ally Switch via TM (Abra and Kadabra are the only Pokémon in Black/White that learn the move via level-up).
Gardevoir is arguably the best Pokémon to teach Ally Switch to, as its Hidden Ability Telepathy makes it a natural fit for Double and Triple battles. This ability prevents Gardevoir from taking damage from its allies in battle.
While Telepathy and Ally Switch don’t interact directly, the combination does allow Gardevoir to become a staple in your multi-battle party.