So at the end of the Emerald Arc (Spoilers I guess)

[Taichi]

When everybody Triples Finishes Archie aka Guile, they never mentioned what move the Electric one was?
The one that Pika, ChuChu, and Pich did? Was it Thunder or something?

 

[Torosiken]

That is....................... a volt tackle....

Its the attack out in emerald game............... so it's the new ultimate attack..

 

[Super Pichu]

I still don't get how Pikachu got it though.

 

[Clover Cat]

I still don't get how Pikachu got it though.

Crystal says, right before the attack, that the two Pikachus as well as Gold's Pichu have also been training with them to learn the new attack. Since Yellow has her little...special ability thing where she can emotionally connect with pokemon, she helps to raise the effect of volt tackle to raise the effectivness of the attack.