The Expression Timeline allows you to apply several expressions to a property such that each expression is active only for a certain period of time. Instead of an abrupt change between the expressions you can also smoothly transition from one expression to the next. Furthermore, you can specify time intervals in which no expression is active such that the keyframed motion is considered.
keyframe several different expressions
on one property
The tool can combine arbitrarily many expressions and works with whatever expressions you give to it.
For the intro scene with TVs on a conveyor belt, generated by two layers of Particular 2.0, I needed a way to smoothly interpolate between straight-line movement and controlled chaos. Expression Timeline worked perfectly, merging the two expressions that regulated how the audio was converted into motion. It then helped integrate those expressions with static keyframes for precise control.Teddy Gage about the work on his showreel
I have to say its revolutionary. Seriously. The ability to blend keyframes and expressions, and mixed sets of expression at will can change the way you work forever. And I'm not usually prone to hyperbole.Teddy Gage on the After Effects Mailing List after finishing this video which uses over 200 lines of expressions code