title: What's the effect of fillMode being kCAFillModeBackwards?
body: <blockquote>
    <p>kCAFillModeBackwards<br>
    The receiver clamps values before zero to zero when the animation is completed.</p>
  </blockquote>
  
  <p>What does the meaning of "clamping values before zero to zero"? A simple example would be appreciated.</p>