If its like melee, SDI recognizes inputs in 8 directions (N,, E, SE, S, SW, W and NW). So the easiest and most effective way to do multiple SDI inputs during hitstun frames is sliding the control stick between the desired direction and the
two adjacent directions. Apparently, SDI inputs must be at least a quarter-circle away. You have to take that into account.
For example, if you wanted to SDI a move to the right, you would go diagonally up, slide to diagonally down, then back and so-on. Moving the stick between directions is a lot easier and quicker than returning to neutral between inputs and makes multiple SDI inputs more feasible. Im pretty sure this works in P:M as well if Im not mistaken.
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