You can only use behaviors in Motion, but they can exist if you're using Motion to create a template for FCPX.
Place a video clip in a Motion project.
Right-click the Opacity parameter, and you'll get a pop-up menu.
Got to Add Parameter Behavior > Oscillate.
For the Oscillate settings, set Wave Shape to Square, and Speed to 50 and watch what happens.
Experiment with other Wave Shapes and Speeds and offsets.
Right-click the text template you'd like to use. Such as Custom or something. "Open A Copy In Motion" Then just apply the modulator to the text layer's opacity. Cmd-S to save, give it a name, put it in a new Custom category so you can find it. I like to keep my custom stuff in a category called Ben's Customs. Here's a transition I made using this very technique. Except if flashes between 2-3 clips. But you can look at how the modulation works.