Similar to other French Bread games, crossup protection is a prominent feature in DFCI. However there are still ways to circumvent the system allowing for very ambiguous left and right mixup. In this primer sides will be referred to as side "A" and "B".
The basis for breaking crossup protection in the first place is getting an assist to hit in a way where it can only be blocked one way. To achieve that we have to first fulfill some criteria. Breaking crossup protection only works after a knockdown, so keep that in mind.
You need to take note which direction the opponents feet are pointing in after a knockdown. In almost all cases it will be towards you, shown as side A. For an assist to only be able to be blocked one way (breaking crossup protection) you have to be on this side. So after knocking them down, you need to make sure you’re on side A while having an assist hit from side A before the opponent wakes up. If you are on side B for even just 1 frame after they’ve woken up, the crossup won’t work.
If you want the setup to be threatening, you must somehow threaten an attack from side B at the same time as you threaten the assist on side A. Here is the most basic way to do this.
After Kirito has jumped over Asuna he has two different mixup options. He can land and press 2A. This will hit meaty at the same frame as Celty 6S connects, which you will only be able to block one way. This is one way to threaten side B. What matters here is that Kirito is able to hit before or at the same time as the assist.
In order to threaten side A you simply double jump back. This results in Kirito being back on side A before the opponent has woken up, which leads to a crossup protection break from Celty 6S.
Notice the way Asuna turns around on wakeup here? This is the easiest way to know that you were too slow in returning to side A, the assist in this example can be blocked by holding either left or right. This is the most basic way to make a crossup break setup scary. In conclusion, you have to make sure you have a way to meaty the opponent from side B while having enough time to make it back to side A before they wake up. By this point you should have enough knowledge to be able to create some setups of your own. Going forward, I will explain how to make these setups even scarier.
Not all setups are created equal. In the previous example with Kirito he barely had enough time to meaty 2A on side B, which meant he had to double jump back for the side A crossup break. With some setups, time won’t be an issue. For this segment we’re going to use a different example setup featuring Yuuki assisted by Wilhelmina. As you can see, we’ll have a lot more time to make our way back to side A in this setup compared to the Kirito one.
This allows us to replace our previous 2A with a jumping button such as j.C. This will make the mixup way more threatening for one simple reason. Neutral jumping into a slight drift then pressing j.C will still hit meaty on side B, while neutral jumping into a full drift will reach side A in time for the crossup break. The mixup becomes very ambiguous and difficult to block.
Example hitting side A
Example hitting side B
Sometimes you may not have time for neutral jump drift to make it to side A in time, but just jumping forward does the trick. In those setups, simply mixing between meaty 2A on side B or forward jump for the side A crossup break is very strong. Some characters, like for example Tatsuya, have a move that can quickly swap sides. This can in certain setups replace jumping back to side A.
While on the topic of Tatsuya, certain characters have moves that will leave them on side B while knocking the opponent down normally. The most common cases of this would be Tatsuya’s 236AB~X and j.214AB. After ending a combo with either of these moves, you’ll already be on side B, ready to mix. The Tatsuya 5AB examples both display this too.
Since Iriya is a very strong and popular assist that gets tons of usage I thought I’d make this short explanation of her assists regarding crossup breaks. 5S plate will not hit from the side you called it on, rather it will hit from the side you launched it from. Attack directions are what matter here. If you hit the table with a falling j.A after jumping over the opponent the plate will still act as if you launched it from side A, even if you were technically on side B as it got launched.
6S is an interesting one. It will act as if you called it on the opposite side, so if you were to call it from side B and then jump over to side A in time the crossup break would actually work. Just make sure the feet are pointing the right way.
There’s a couple of other assists in the game that act similarly to Iriya 6S, notably the 2nd hit of Rusian 6S as you could see in the aforementioned Tatsuya 5AB setups. A more detailed write up of Tatsuya and Rusian setups can be found here.