Think of the migration of a common-depth-point stack as downward continuation in -space. Ordinarily, velocity increases with depth. As the downward continuation proceeds, the velocity cutoff along the evanescent line bites out more and more area from the -space. Energy beyond this cutoff does not fit the primary wave-propagation model, and it should be suppressed as soon as it is encountered. Such noise suppression can lead to a large drop in total power at late times.