The reason she isn’t being splashed is because she put an image of herself there while her body is somewhere else. She did that because Fai is cursed to kill anyone with stronger magic than himself, and Yuuko knows that. So she’s hiding so Fai won’t go ham and murder her right there.
Aaaahh, yes, I do remember that now that you mention it. This was what all went down in the Infinity arc, wasn’t it?
From what I remember, Fay Wang cursed Fai because he wanted him to kill Syaoran, when Syaoran’s power surpassed Fai’s…but Sakura interfered and ended up getting stabbed instead. I think, anyway.
And yes, smart move by Yuuko then. Which brings into question just how much Yuuko knows about everything that’s about to unfold for the Tsubasa crew. My guess is: most of it.