YoTricks.com

Rock the Baby is one of the most famous, but also one of the easiest yo-yo tricks for a beginner to learn. Now, Rock the Baby is a picture trick which means that you make a picture out of the string. The easiest way to learn these types of tricks is to just let the yo-yo hang down at the bottom of the string without spinning and that will give you all the time you need to actually work on and practice the trick.
This trick, like I said, is pretty simple. The first thing you do is you take your opposite hand and grab the yo-yo string right about in the middle, and then you want to take your yo-yo hand and pinch the string just a couple of inches above the yo-yo. Next, take your yo-yo hand and put it right under the yo-yo and point your fingers forward and that makes a cradle. You just rock the baby through a couple of times. When you drop it, if the yo-yo is still spinning quickly, that will give the yo-yo some slack in the string… it should come right back to your hand.
In full speed the trick looks like this.