I was just facetiming my daughter and she made a heart with her hands and animated hearts appear on the screen. Then I made the peace sign and balloons floated in the screen.
You can do it manually with the options that appear when you touch your self video insert.I just FaceTimed my parents and did the peace sign, worked great. The cool part is the real time cutout that puts balloons *behind* you as well as in front.
Can't really do the heart sign when holding the phone with one hand. I think they should have a button that lets you manually trigger these reactions on the iPhone -- in fact that's what I was searching for before remembering it's triggered by hand motions. Another issue is I don't know which reactions exist because there's no guide anywhere, effectively making this feature an easter egg.