From the CAMediaTiming protocol documentation:
Determines if the receiver’s
presentation is frozen or removed once
its active duration has completed.
This determines what happens at the end of your animation. By default, it is set to kCAFillModeRemoved, which means that the animation changes are undone when the animation is completed. If you switch it to kCAFillModeForwards, the changes caused by the animation will hang around.
Note that you will also probably need to set the removedOnCompletion property to NO to prevent your animation from being removed, which would make this setting ineffective.