I get a lot of questions about this, no better source than Annie:
http://media.crossfit.com/cf-video/CrossFit_KippingPullupStep1.mov
http://media.crossfit.com/cf-video/CrossFit_KippingPullupStep2.mov
http://media.crossfit.com/cf-video/CrossFit_KippingPullupStep3.mov
http://media.crossfit.com/cf-video/CrossFit_KippingPullupStep4.mov
Enjoy.