Okay there are like 5 hit boxes. Here's a sick MLG visual image.
........-- 1
....-- 2
..-- 3
....-- 3
.......--2
On the very top of the tail dunks in a reverse fashion. It pulls the enemy toward you in a downward angle.
Right below that hit box is the "Dunk Box" as dubbed by myself. This box does the "dunking" in a diagonal fashion away and down.
Every other hitbox sends straight up essentially except for the one like really close to Pikachu's butt as I am now dubbing "Butt Dunk". But it also works in the "reverse" fashion. But it's incredibly unsafe and you won't find many uses for this particular hit-box.
1. Reverse Dunk (Tip)
2. Dunk (Just below the tip)
3. Lift up (Pretty much every other hit box)
I'm not sure if I'm entirely correct, but it feels like this when I play, and I have a pretty good consistency of hitting both the reverse and the standard dunk.
Battle Tips:
1. Reverse Dunk - Is a neat combo that leads into Nair on most characters. It's beautiful. 10/10 Would do again.
Example:
http://www.youtube.com/watch?v=xa7MYdNq3mQ
It's also good against Peach (imho) if you can land it consistently as it's relatively safe, and gimps her. R.I.P.
2. Dunk
As you probably know, this is very useful against everyone and can also be seen in the Axe vs. Silentwolf x4.
This is super awesome, because it allows you to combo characters into UpSmash like Marth/Cpt. Falcon etc. I think it can also be a set up for Jigglypuff, etc. It forces a tech chase at the very least, and that in itself is useful.
3. Lift up
This is good for cross-up Nairs on most opponents and allows you to combo them with your Nair or whatever you feel like. Personally I always end up doing another U-Air into itself a few times before figuring what I want to do, depending on the opponent.
I didn't intend for this to be a Pikachu 101 session, but I wanted to cover common uses, etc. that give you a time to use it, etc.