Dolphin kick drill that strengthens core muscles

At Glenn Mills' site; GoSwim.tv, I found this dolphin kick drill that strengthens core muscles. There is a Flash video in the upper right you can click on to see it demonstrated. Snippet:

• The buoyancy of the pull buoy adds an extra bit of resistance, and makes you work more intensely with your core. The added resistance of the pull buoy forces you to be super aware of how you position your body in the water. Notice that the swimmer in the video keeps her hands, arms, and head in super streamline. She forms a stake with the front part of her body, and drives the stake through the water using her core and her legs. ... [Link]