That is called the HOM or Hall of Mirror effect, it's just a place where nothing gets redrawn. Now about leaks and rad. In order to have a level correctly lit, there can be leaks in the map. This is, however, not supported by 'our' compile tools. Although it's highly unlikely it is possible Valve used other compiletools which took care of leaks right away (by just ignoring them). If you are using ZHLT you can also create the HOM effect by null-texturing a wall. I believe your map will still be correctly lit after you do this.
One 'hack' which uses HOM is motion blur. Make a map, make all the brushes which touch the void func_walls and render them transparant (texture). Now surround the bunch with a null textured 'skybox' (nullbox rather) or place nulltextured brushes to seal the entire map from leaks, and play the map, just have a look at what happens.