Awhile back, Zero told me that during some circumstance, Sonic's iSDR can beat tornado, but I don't remember the conditions.
When its completely fresh it beats nooby low tornadoes. So basically useless, because its its extremely difficult to follow up against MK so you won't be able to land 9 attack between each iSDR, but its nice to know i guess.
Especially if you just wanna land the first hit and ***** it out.
Yes, it does beat low tornados, but it still goes through high ones, and even though it wouldn't hit him, you wouldn't get hit either.
Basically you have to hit him almost as soon as you release the iSDR for it to beat nado.
actually, the timing of the release, charge, or connection doesn't matter, it just has to be a fresh iSDR
I think the tornado has to be on the ground or some impractical nonsense like that.
All in all, the conditions are, fresh iSDR, that's it.
If the Nado isn't low enough for you to hit him, he won't take damage, and neither will you, you also won't stale the iSDR, so it resets to a neutral position, which is fine if you're *****ing out like Tesh mentioned.
In short, you shouldn't be focusing on the iSDR much, but if you can keep your iSDR fresh then you've got a great tool in the fact that you've got a move that guarantees a safe escape regardless of what the opposing MK does.
I also gave a nice list of moves+positioning that you can use on MK as safe follow ups and crap to refresh it, if you *did* care about keeping the iSDR fresh.
Edit:
http://www.smashboards.com/showpost.php?p=14566124&postcount=21377 <--theorycraft on the YI MK MU