I'm assuming what you're talking about is the horizontal 1020 spin he does. That is Pikachu's Back Aerial. If Pikachu is in the air and facing Left, hit Right and A to do it, vice versa if he is facing Right. But in all honesty, the only time you would ever need this move is if you had no other option left, because of the huge lag problem the move faces if it hits the ground, PLUS speaking of damage, unless every hit connects, the damage is hardly move-worthy. You are much better off with forward Aerial A leading into a Down Smash, or even his Neutral Aerial A or Down Aerial A if you're not planning on hitting the ground anytime soon.
Don't think I'm discrediting his Back Aerial A though, I'm an avid Pika player and I use this move maybe 3 or 4 times in a match tops, ESPECIALLY if the opponent doesnt see it coming
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If you still want to use this move and need a little practice. 1) Hang on a ledge. 2) jump off the ledge. 3) Tilt the control stick back toward the ledge. 4) Hit A and use his Back Aerial.
Hope this helped you out!