It really depends on the type of knockdown that you get. If you’re thinking about doing it from a sweep, it’s manual timing since you don’t have time to whiff something and then jump in safely. From a throw in the corner, you can whiff st.LP/LK and to set up a “safe” cross-up attempt, but it’s the button press timing that would determine if you cross up or not.
If you press early, the hit will be towards the front and you’ll land in front. Delay by a couple frames and it’ll cross up. When Mago does it, it doesn’t seem like he readjusts spacing from an unteched or untechable knockdown, he just gets the timing of the press at the right moment so that Fei’s j.MP hitbox doesn’t allow him to move past the opponent’s (if he wants a frontal hit), or once he’s moved just behind the opponent already (if he wants a cross-up). That seems to work well for myself too, but it takes practice to get it down. Maybe there’s also a set-up that I’m not aware of though. Hoping someone can enlighten me if there is.