The ads appear at the bottom of a video shortly after the clip starts to play and disappear after 10 seconds. They resemble the TV-style ads that often feature the image of a celebrity walking onto the bottom of a TV screen for a few brief moments. YouTube executives said Tuesday that extensive testing showed that viewers rejected any format that forced them to sit through a commercial prior to a clip being played, a process known as a pre-roll.
Using overlays, executives said, was the least intrusive way to get a message in front of their audience, or so they believe.
See also The future of video ads on Webware.