I swear this like the third time I've seen this asked haha
Charging the second 2 in the 212 string does nothing... I'll copy paste what I wrote in the General Discussion since I'm too lazy to write anything else...
"Charging the second 2 in the 212 string allows you to sometimes drop your combo making your opponent think you're horrible at this game and underestimate you... Totally high level mind games brah!
But seriously I don't think it does anything... It doesn't become unblockable, it does the same damage and opponent can techroll/wake up.
You'd think maybe you could use it as some kind of ghetto frametrap but, when doing this to the Sub-Zero AI in training I've gotten Ice Cloned and B1'd, he also was able to jump back and he could probably just D4 me. Then again even if you could... Why would you? Not charging puts you in the same position you were even if you did so why bother adding risk to it?"