A few moments after the video beings playing you may see it resize and thumbnails populated below the video. Thumbnails are only displayed when viewing a single channel and if the service has been enabled by your MIP administrator. The Thumbnail button at the lower right hand corner of the Viewer sets thumbnail time frame. With a recorded program you can use Storyboard to quickly move forward and backward in time. With a live program, obviously, you are only able to move backward.
At the end thumbnails are automatically updated with the next time increment.