The aft nacelle associative is a field pattern associated with warp modulation.
This field pattern was of a higher energy level than the forward saucer associative.
When Geordi La Forge was traveling in the type 15 shuttlepod Onizuka to Risa and wanted to pass the time, he played a trivia game with the computer. Listing all field patterns associated with warp modulation in order of decreasing energy was one of the questions. (TNG: "The Mind's Eye")