expressions in animation are a fast way of creating animations that would be hard to do with keyframes.

Alt then click on the stopwatch to add expressions

“when you want to create and link complex animations, but would like to avoid creating tens or hundreds of keyframes by hand, try using expressions. An expressions is a little piece of software – much like a script – that evaluates to a single value for a single layer property at a specific point in time. Whereas scripts scripts tell an application to do something, an expression says that a property is something.”

LoopOut(“cycle”)

wiggle(freq,amp)