On the 3DS, smashing the stick forward, rotating to another direction (or returning to neutral) and then hitting A would result in an instant dash attack, or IDA for short. What you've discovered is that, due to the way the C-stick is interpreted, up and side flicks don't work.
This is probably because the dash is meant to be cancellable into a jump (hence why DACUS exists) and its initiation must cancel into a forward smash to allow players to perform the move by stick (hence why you can't throw out a dash attack from standing by normal means. I guess, since back smash is the same move as forward smash, the game treats them the same for this purpose.
The game cannot, however, do anything with a down smash input during the dash initiation period, meaning that it is ignored and only the attack input from the C-stick can be used. Since smash attacks with the C-stick result in a negation of all main stick input in Smash 4 (until both return to neutral) no forward input will be seen by the game, meaning it will register a neutral attack during dash instead of a forward smash.
I'd be curious as to what happens if you set the C-stick to tilts but, as is, you have not discovered a new technique, only a new and easier means of execution.
