What does ApplyBuiltInRootMotion do?

I was getting some pretty horrible results by doing this:

Code (csharp):

1. void OnAnimatorMove() {
2.     animator.ApplyBuildInRootMotion();
3. }

The movement was stuttery, and moving the y-position of the transform (to follow slopes) caused it to slow down a bunch.

When I instead do this:

Code (csharp):

1. void OnAnimatorMove() {
2.     transform.position = animator.rootPosition;
3.     transform.rotation = animator.rootRotation;
4. }

Everything looks great. The movement is just as I expect it to be, and there's no stuttering.


So what is in ApplyBuildInRootMotion is doing that could affect the character in such a way? Does it assume that I don't move the transform manually at all? This is Unity 2018.4, if that matters.