After getting grabbed by IC, is there always absolutely nothing you can do?
Depends. If they're good, you're pretty ****ed unless you're at a low percent. If it's a low percent you have a good chance to get out if you're good at mashing.
Also, smash DI is using the c stick to DI, and regular DI is using the regular stick correct??
Not quite. Better educated/more experienced people correct me if I'm wrong please.
Okay if I remember right it goes something like this. Every attack has hitlag (or most? I'm not sure maybe I'll have to check PSA or something to see, but whatever...). Hitlag if you don't know is when both the attacker and the attacked are frozen after the hit. Almost like a dramatic effect. You can notice this a lot with Zelda's death kick and The Knee.
If you use your control stick, you can "teleport" a small distance in that hitlag for every input. (most attacks have the same amount, but Shiek's Ftilt only has half the normal amount, and Zelda's death kick and Marth's tippers can't be SDI'd at all) So Quarter Circler DI is when you move it between the three notches of your analog stick to get lots of inputs fast. And if you're holding a direction on the analog stick while SDIing, every time you use the Cstick it adds another "input" (although you have to make sure to let the Cstick back to center after every input).
It's difficult to explain, but I think that's pretty much correct. People correct me if I'm blatantly wrong
What other uses are there for smash DI other than breaking out of Pikachu's dsmash?
LOTS of stuff. Like if you're good at it, there are so many more things you can do and not be afraid of. Any multihit attack you can get out of with good SDI (Wario's DAir (I think. Idk he has so much aerial mobility he might be able to follow your DI to keep hitting you with it lol), G&Ws NAir and BAir, etc). It's really, really useful.
When sucked into Metaknights tornado, do you smash DI or DI regularly?
I try, but I suck at SDI