Dr. Turell, - What they're talking about is the sun bobbing through the central plane of the milky way. When it's on the topside of the disc--that's when we're bombarded with more cosmic rays, due to the backlash of a cluster we're hurtling towards. I'm not familiar with the geologic timing of the mass extinctions, so I'm not too informed about the breadth of the article. But it's a highly imaginative idea.