http://youtube.com/watch?v=0fKZ7SGx83U
As quoted by N64, who starred in the above video (he posted as MentalFly):
"... the uair has an interesting hitbox. It's best to experiment on your own and get a good feel for what part of the tail, and in what part of its swing, will send the opponent in the desired direction, but here are its general properties:
(I'll use a 0'[behind pika] to 180'[infront of pika] scale, with approximations, to represent the tail's position.)
0'-60' = Pops them up and slightly back.
61'-90' = Reverse spike.
91'-120' = Forward spike.
120'-180' = Pops them behind you and slightly up.
As far as how the part of the tail affects the knockback on the opponent, I'm not quite as sure. It seems that it at least slightly changes the trajectory and overall knockback, increasing the farther our it hits."
I wouldn't have known that if it weren't for him.