The words appear one at a time at the end of the line, and when a line is filled, it rolls up to make room for a new line.
Older decoders can only display roll-up captions at the bottom of the screen.
Newer ones can place captions wherever the captioner wants them. The entire caption appears, all at once, anywhere on the screen. When a pop-on caption appears, all captions previously on the screen are erased. They are not used much yet, except for commercials and special effects. Caption placement is a tricky part of captioning.