Here, for your reference, is a visual representation of each transition, and what editing circumstances might call for it.
Billow might also be called “Polka-Dot,” “Acid Drops,” or “Expanding Swiss Cheese.” As the clip progresses, a fleet of, well, flying holes descends on the first clip; you can see the second clip through the holes. The holes gradually grow until they occupy the entire frame—and presto, you’re now in a new scene. You can use the directional arrows to specify a general direction for the flurry of UFH’s (unidentified flying holes).
It’s kind of hard to imagine when this transition would feel natural, except perhaps in documentaries about cellular reproduction.
This effect, called iris close (iris open) or iris in (iris out) in professional editing programs, is a holdover from the silent film days, when, in the days before zoom lenses, directors used the effect to highlight a detail in a scene.
It creates an ever-shrinking (or growing) circle with the first clip inside and the second clip outside. It’s useful at the beginning or end of the movie, when the second clip is solid black and the subject of the first clip is centered in the frame. In that setup, the movie begins or ends with a picture that grows or shrinks away to a little dot. (If the subject in the center waves goodbye just before being blinked out of view, this trick is especially effective.)
The crossfade, or dissolve, ...