These two expressions create a “ follow the leader” effect. One works from the top down, the
other from the bottom up. Only use one.

From Adobe's LiveDocs:
You can use any expression in place of the wiggle expression used here, to begin and end any expression’s influence at a specific time.

How to create text that flows with water or fabric.
By Trish and Chris Meyer

Sometimes though, it would be extremely useful to be able to get the wiggle motion to loop. Here we'll take a look at a way to accomplish this by using a bit of clever math and taking advantage of one of wiggle()'s seldom used parameters.

Often very useful to wiggle only horizontal or vertical