Music is math.
|--Michael Sandison and Marcus Eoin (Boards of Canada)|
Expressions open up a whole new realm of functionality in After Effects by enabling you to link elements of your project together and apply logic and bits of math to the connections.
Here is where the standard disclaimer is usually added about how artists don't like logic and bits of math and where I promise to shield you from it as much as possible. But I don't buy the underlying assumption that because you are a visual artist, you don't want to be bothered with pesky numerical data, technical details, logic, or math.
Admit it: You deal with that stuff all ...