If there's a cute visual mnemonic for this one I'd love to know as much as anyone, because reverse tailspike is stronger than forward tailspike, which means that it's Pikachu's strongest horizontal kill move, and thus... pretty good.
The first thing to remember is that up-air has four hitboxes which go in this order (Pikachu is facing right):
2 3
1 4
Hitbox #2 is reverse tailspike. Easy way to remember is that the top hitboxes hit
away from Pikachu and downwards, the bottom hitboxes hit
towards Pikachu and upwards. Often when I'm aiming for #2 I'll hit with #1, knocking my opponent straight up, and it's because I pressed the button too late. But if you press the button too early it's a whiff, as Axe demonstrates here:
https://www.youtube.com/watch?v=eipt1gDolbE#t=0m46s
In fact if he had hit that tailspike it could have evened up the game; instead he took 50 damage and nearly lost a stock for it. In other words, RTS is haaaaard, even for really good players. So my approach is: just keep practicing, and try to hit it over and over again.