MOUSEOVERstarts the movie start when you put your mouse over it.
For example, this code
<IMG SRC="../graphics/moonflag.gif" DYNSRC="../graphics/moonflag.gif" START=MOUSEOVER ALT="Astronauts on the moon">
starts the movie when you put your mouse over it:
The idea is that
MOUSEOVER makes the movie wait until the user gives some kind of "start" command. Unfortunately most people don't catch on that they need to put their mouse over the image so it generally doesn't work.