What's the effect of fillMode being kCAFillModeBackwards?

Question

kCAFillModeBackwards
The receiver clamps values before zero to zero when the animation is completed.

What does the meaning of "clamping values before zero to zero"? A simple example would be appreciated.

Answer 1

Since no one gave me an answer and the documentation of fillMode is really bad, I did a research and re-document it here.

kCAFillModeRemoved

The receiver does not appear until it begins and is removed from the presentation when it is completed.

kCAFillModeForwards

The receiver does not appear until it begins but remains visible in its final state when it is completed.

kCAFillModeBackwards

The receiver appears in its initial state before it begins but is removed from the presentation when it is completed.

kCAFillModeBoth

The receiver appears in its initial state before it begins and remains visible in its final state when it is completed.