I rather learn which way to DI. I can DI most attacks, pika's down smash, peach's down smash (basically same thing), MK's tornado (depending on the direction) and other attacks. And zelda's is one of the few that i don't know which way to DI out of, i know it can be done cause i've pulled it off with luck, but it's hard when i intent it.
And from my experiences it doesn't go back and forth all the way, it also goes up and down. Yeah, double trouble.
The following is the result of cumulative hours in 1/4 speed experimenting with the effects of SDI, QuarterCircle DI and CrazyAss DI on attacks with "particle type hitboxes"...but it could still be wrong:
Personally, I think that Zelda's Upsmash isn't DIable in the sense that there's a surefire way to DI out of it. I think the same goes for Metaknight's tornado. In the case of the Mach Tornado, I find that I can escape from it at the same level of difficulty if I don't DI as if I do try to(And I've tried Everything). However, I did find that lighter characters are randomly kicked out far more often than, say, Ganondorf
.
As far as Zelda's UpSmash goes, I've only tried figuring out the DI with heavier characters(Ganon), and I found that it's just impossible. Even though there's a definite break before she brings her hand back to its starting position, The fact that you can't DI these minute particle hitboxes AND the fact that they have crazy hitstun for how weak they are individually just makes it impossible to escape.
Zelda's F-Smash on the other hand, can be DIed. However, the direction that you DI depends on how far away from her you are. If you're really close, try to SDI up and towards her and QuarterCircle in that direction. If you're farther away, do the opposite(up and away from her).
I'll buy a cheeseburger for the first person who can consistently DI out of Zelda's UpSmash and put it on the forums.