Oh no, the secret for me grabbing random things (like marth's tippers on my shield) is out.
SDIing by holding a direction and hitting up on the C-Stick is a hilarious concept.
I am going to write an indepth detailed thread at some point soon on why this is the case but essentially:
If you hit a direction on the cstick while holding a separate direction on the analogue stick, it inputs one frame perfect input immediately after the cstick input assuming the two sticks are split between at least two cardinal directions (up down left and right).
For example: Holding right on analogue stick while tapping up on cstick is seen as:
Frame 0: Holding Right and Inputting Cstick up
Frame 1: SDI Up (from the cstick, thus inputting grab)
Frame 2: SDI right (because you were holding right, it frame perfect inputs the held direction on the analogue stick)
Frame 3: Null input
With ISSDI, in order to input ISSDI without rolling, one needs to be frame perfect. However, I'm FAIRLY certain that with the limitations of control sticks (ie, being too top heavy so it doesn't reneutral fast enough) that it's next to impossible to analogue input an ISSDI without accidentally buffering a roll out of shieldlag. Don't quote me on that; it's just speculation on my part because when I first started playing I was told I could ISSDI MK's Tornado, only to consistently roll into it.