Why does this buffered input result in an up smash?
Normally, three things need to be true to use a smash attack:
- You moved the control stick a certain distance.
- The control stick was neutral a short time ago.
- You pressed the attack button within a certain time of both 1 and 2 occurring.
However, up smashes have an additional condition, partially for "convenience" and partially because there's not much alternative:
- Either #2 is true, or you're in jumpsquat (the short time when your character is bending their knees preparing to jump) when you press the attack button.
Therefore, if you want to buffer an up aerial from the ground, you have to move your stick so it passes the up aerial detection threshold but not the up smash detection threshold. This shouldn't be too difficult; in Brawl the up smash threshold is 66.25% while the up air threshold is 25% (I don't have the SSB4 numbers, but they're probably not much different), so putting the stick at about 50% between neutral and full up should get you an up air without chance of an up smash. (There's a chance that the game will eat your attack button input anyway, in which case you're out of luck.)