Not sure if this is already known, but I just found out today that you can use thunder to reverse Pikachu's horizontal momentum (vertical momentum is unaffected). If you activate thunder while moving horizontally in the air and then immediately jam the analog stick opposite to the direction of Pikachu's movement, Pikachu's horizontal momentum will be reversed (the timing is a bit tricky, though...). If anyone wants to try, I find it pretty easy to do in practice on 1/2 speed by jumping forwards and then pressing b + quarter-circle back.