A transition is an interim effect that runs between the end of one display effect and the beginning of another. By default, a new effect will simply kill the old one and overwrite it with a new image.
Transitions are scheduled – a handler is installed that runs the next time that a new image is shown. Instead of showing the image right away, the transition handler kicks in and displays some combination of the present view and the new one. When the transition finishes, the display should consist entirely of the new image.
Currently, all DMD transitions are contained in common/dmdtrans.c and segment transitions are in kernel/segment.c.