when you are moving your head back and forth slippin and bobbing the purpose is to not keep your head in one spot, your head is alot easier to hit when its not moving than moving. Also it keeps you focused and ready for example if your not moving your head your reaction time to someones punch will be slower, when your head is moving you have that headstart. There is alot of rythm and timing to it so it takes work. To help with you head movement while punching put a mirror on the wall level with your head hieght while your in your stance and shadow box. While you are shadow boxing make sure your head stays out of the mirror for example when you throw your right lean with it a little and even when your not punching keep slipping and bobbing. This has helped me with my overall head movement.