I get that part, but does that mean that using the control-stick and the c-stick together makes no difference and does nothing more than just using the control-stick?
Using both has absolutely no effect on DI?
Since rolling the cstick doesn't work for SDI (might work in Melee? not sure) you'd have to tap it repeatedly,
really quickly, inhumanly quickly, to see a noticeable difference from using the c-stick. Technically it might make a difference, but it won't be by a significant amount.
For
ASDI, however, the cstick is useful. It lets you ASDI straight down (pseudo CCing, where you aren't actually crouching but still ASDI down into the ground to land and cancel hitstun) since it takes priority over the control stick for ASDI, while inputting a trajectory DI on the control stick different from the angle of the SDI, allowing you to DI an upward+away sending move downward+away, optimizing your downward trajectory DI so that on the first frame (the frame of ASDI) you don't travel as far upward so that ASDI into the ground is effective at higher percents, enabling you to input a tech and survive some strong hits at high damage if see it coming and are trained enough to react quickly. It's called double stick DI or just DSDI like chevy said.
It's helpful for characters that will often be put in the position of a lot of lag at high damages, especially characters like sheik/marth/Mario who might have to up-b onto the stage, and experience enough lag for the opponent to land a strong hit. DSDI into the ground and hitting the shield button will often enable them to survive. Jiggs uses the same thing for missed rests or if the opponent spawns again after a successful rest with enough time to retaliate.
It can also be dangerous though, if the hit is strong enough to lift you too far off the ground to ASDI down in the first frame of knockback, you'll be sent at a much lower angle offstage. You may also miss the tech, so even if the move isn't TOO strong to ASDI into the ground, you could get knocked down and slide across the stage and then way off of it, with horizontal momentum and no vertical momentum other than your falling, leaving you at an even worse position.
So yes, adding the cstick can help in the long run if you put in the time and effort to learn to DSDI, but the overall difference will be miniscule because opportunities to use it are scarce, and there are ways of beating it fairly easy, such as going for vertical kill moves which beat pseudo CCing even at moderate percents because they send comparatively high off the ground on the first frame. How useful it is depends mostly on your character, from a range of "nearly useless" up to "somewhat useful". Don't stress over needing a cstick for it. Hopefully somewhere in here I answered your question by way of this much info.
