Small words (kind of), big effect. Occlusion culling is a feature that optimizes the rendering of 3D scenes. If an object in a scene is blocked by another object, it would be a great waste of GPU-time (and bandwidth) to nonetheless render this blocked object. Besides needlessly increasing the triangle count of a scene, the (invisible) object would also need to be textured or have pixel shader effects applied to it. To reduce this waste (overdraw), both ATi and NVIDIA have developed techniques to detect these hidden objects and exclude them from the rendering process.
With its Villagemark, PowerVR created a benchmark that tests the performance of these culling algorithms. Originally, it was meant to showcase the advantages of the PowerVR tiling architecture of the Kyro line of cards.
The results speak clearly. Even though the Radeon 9700 has a higher triangle throughput to begin with, we can conclude that ATi's Hyper-Z implementation does its job well.