Here is what Fly posted:"Some of you already know about this from Twitter, Twitch, or talking to me elsewhere, but I haven't made a post about it yet and I think it's pretty useful in some circumstances:
A lot of the time when ICs are synced and have the same shield size, an enemy's attack will hit both ICs' shields at the same time. Popo and Nana hence exit shieldstun at the same time. Since Nana reads inputs 6 frames late, this means that if I input a command within 6 frames of shieldstun ending, Popo will ignore it since he's still in shieldstun, but Nana will receive it after exiting shieldstun and hence act on it. There are a lot of things you can do out of this, but the most useful one to me seems to be telling Nana to jump and blizzard while Popo continues shielding. Ice block instead of blizzard seems better in some circumstances, as well, like when you're confident the enemy will stay near the ground and not throw out a move that will clank with the ice block, or when you don't have a guaranteed punishment OoS and the enemy is outside of blizzard range.
More explicitly, the main application I see of this is to punish or reverse the pressure caused by high damage moves that ICs have a difficult time retaliating against out of shield. For example, Falcon kneeing a shield over and over can be punished with this; it can also be used to retaliate after Ganon fairs your shield, which can be a pretty threatening situation, especially when you're cornered. You can even squeeze a blizzard between two Marth fairs, but the timing for that is pretty strict and much more difficult on your end than Marth's.
In a lot of ways, this is like doing belay OoS -> blizzard after an attack hits your shield, but other than being more difficult than that, it's essentially strictly better. For one thing, doing belay OoS requires inputting the belay after Popo's shieldstun ends, meaning Nana isn't going to jump and start a blizzard (or whatever else you might want to do) at least 6 frames later. With this shieldstun desynch, you can have Nana jumping up to 6 frames before that. Aside from that, the solo belay animation is terribly laggy, and you generally can't expect to get any sort of punishment off of the blizzard (or ice block/etc). With the shieldstun desynch, you can have Popo act OoS as soon as you want him to. To give a sloppy example of this desynch in action,
here's an instance of me using it at TNE (the time being 8:16 if the timestamp doesn't work for you). I hit down after shieldstun ended and hence accidentally spot-dodged, and reacted poorly after that as well, but I hope that clip suggests how this can actually be useful.
The last thing I'll say about this is that the timing varies a ton depending on the move, and also a bit on the move's staleness. When a move hits your shield, you first go through hitlag (almost always just a function of move damage) and then go through shieldstun (purely a function of move damage), so the time you exit shieldstun tends to vary a lot depending on the damage of the move hitting your shield. This means that the 6 frame window in which you want to tell Nana to jump occurs in pretty different places depending on what hits your shield, so it's really hard to do this on a whim. I recommend learning the timing for specific high-damage moves which are difficult to punish OoS, such as the aforementioned knees and Ganon fairs. There are plenty of other examples across the cast."