Mamoworld shows how separate dimensions work in After Effects and why.

Maybe you’ve added keyframes in After Effects and then hit Separate Dimensions, only to find that things have changed with your animation. The keyframes so lovingly put down now have a different interpolation setting to them — But why? Check out this in-depth explanation from the folks over at Mamoworld.

Separate Dimensions in Ae.

When you separate the dimensions in Adobe After Effects, it will expand a single position property into a layer with two separate properties for the x and y positions. “Most people assume that this is just a cosmetic difference in the user interface, but it is much more than that!”

It turns out that it will completely change how After Effects interpolates between keyframes. The short video expertly explains it all. It’s important to note that when they say “timing” in the video, they really mean “spacing” — The timing (keyframe positions) remain unchanged.