title: How would a cutting effect be implemented on iPhone?
body: <p>The "cutting" effect I am talking about can be seen in the famous iPhone game "Fruit Ninja" and "Cut the Rope", that is, when you move your finger across the screen, there is a trail following your finger to simulate the knife cutting effect. </p>
  
  <p>I am wondering what's the name of this kind of effect and how it is implemented. 
  Could anyone give me a hint?</p>