How do i get rid of the ugly cross splitting after seperating my faces I'm making a Faceplate for my Self FNAF character and while trying to seperate the mouth plates from the head plates this problem happens. As you can see this is before i hit the Y button to seperate the mouth faces from the Top FacePlates And this is AFTER. i hit the y button to seperate. Do you notice how that looks just weird. I Dont want that, I Need it so it looks flat on the bottom of the top faceplates. But the top of the Mouth faceplates look like they do with Y pressed now.
you can create Edge Creases,

Shift + E in the two edges (marked by purple Line)
It should work



Wiki about subdivision surface modifier and Edge Creases