Once you've wound the complete skein of yarn, just slip it off the ball winder. You'll notice that you haven't really made a ball, it's something a little flatter and that has a hole in the middle.
The hole helps prevent you from winding the yarn too tightly, and it diminishes as the yarn sits or as you work with it in most cases.
Now you have a ball of yarn that you can work with from the outside or use as a center-pull ball, which some knitters prefer. You could even knit with both ends at the same time if you needed to!
