To better understand this, suppose one has two screens, called A and B, with the ability to instantly switch the display from one to the other. The initial frame is loaded in to screen A and B. Screen A is displayed. The differences between frame 1 and frame 2 are calculated and altered in screen B, which is then displayed. Then the differences from this and frame 3 are used to alter screen A, which is then displayed, and so on. Note that frame 2 is stored as differences from frame 1, but all other frames are stored as differences from two frames back.