I like to think of it in connection with a forcing function.
Say you push and pull on a spring in a sine-like fashion. The end of the spring you hold will be in perfect sync with your hand, so there delta = 0. The other end of the spring will not typically be in sync, there may be a delay. So when your arm is at maximum extension, the end of the spring might be coming back in - that's where delta isn't zero. Look on the web for lots of videos on forced harmonic motion.